It’s especially notable for Disney movies, but also happens in many other forms of media: mothers of main characters, especially teens or young adults, tend to turn up dead either before or during the story. That’s a tired trope by now and there are ways to avoid that and still have the main character have their adventure.
First of all, why do authors kill off the mother? In some old fairy tales, the mothers are replaced with the ‘evil stepmother’ - which mostly was a move by Jacob and Wilhelm Grimm, who didn’t like the idea that the birth mother would try to kill their own children, so they made the mothers in stories like Hansel and Gretel or Snow White stepmothers instead. In other stories, the mothers are just dead and thus can’t help their children - or only through supernatural means.
Parents are a liability, especially to characters in stories for teens or in young adult novels - they may keep the main character from doing something stupid, but necessary for the plot. The easy solution to this would, of course, be ‘not to do something stupid, even for the plot’, but that’s rarely feasible. Besides, people (young and old alike) do stupid things all the time, so it’s not as if those protagonists are alone in that.
While the fathers often show little interest in their children, it would be weird for most people to write a mother like that. Mothers care, that’s one of their regular traits (although not necessary always true in reality), they keep an eye out for their children and that means they wouldn’t let their child go off on a quest or decide to travel with a suspicious stranger. For that to happen, the mother needs to be removed.
As mentioned, though, there are quite some ways around that. The “Don’t tell my parents…” series doesn’t remove Penny’s parents most of the time, unless it’s short-time for a few days when they’re travelling to meetings or suchlike. Instead, Penny is forced to work around her parents to do what is necessary. Since she’s a genius-type character, that’s a fitting solution - she has the brains to do it and this is part of ‘showing, not telling’, too. In the same series, one of Penny’s friends has uncaring parents who neglect him, anyway, so he doesn’t have to find solutions to disappear on them - they don’t care where he is or what he does.
The first Artemis Fowl novel, unlike the movie supposedly based on it, removes his mother from the equitation through sickness, which means she doesn’t have an eye on his actions, but which also motivates one of his actions later in the story. Later on, he carefully works around his parents, something his genius trait makes possible - very much like Penny’s, too.
If you’re working in a fantasy or sci-fi setting, sending a teenager or young adult on a quest alone might even be a regular thing and their mother wouldn’t mind them going on it. That’s the advantage of speculative fiction - it just depends on what kind of world and what kind of family you are showing.
Especially today, with many working mothers, it’s not that hard to give teens and young adults the necessary freedom to make stupid decisions for plot reasons. Teens and above aren’t exactly the kind of people everyone would want to watch around the clock, so they can do what they need while they’re not watched. Teens are also traditionally good at sneaking out of the house at night and doing stuff they’re not supposed to do. It’s an extra challenge, but will also provide the story with extra tension - will our protagonist manage to keep their parents in the dark about their midnight trip to the suspicious carnival or will they be caught?
There are many reasons why the mother doesn’t stop her child from doing something stupid (or at least unwise). She might be otherwise occupied (see the working mum) and thus not around while her child is planning. The child may be keeping the knowledge from her, claiming to stay with a good friend for the night when really going to that suspicious carnival. The mother might not care about the child - which isn’t nice, but quite realistic. There’s also always the option (with older teens and above) that the parents go on a short trip, leaving the child behind to look after themselves - which then opens the door for going to that suspicious carnival.
There’s also the ‘boarding school’ route: the teen or young adult in question isn’t at home to begin with, but at a boarding school, college, or another place where teens or young adults would stay. That opens the door for nominal guardians, but they might want the protagonist to do the stupid thing or might simply be otherwise occupied (since they have more than one, two, or three children to keep an eye on).
In young adult fiction it is especially unnecessary to kill off the mother - the protagonist is nominally adult, so they are supposed to make their own decisions and their parents or other guardians can’t stop them from going to that carnival, anyway.
In those cases, I think, mothers might serve as mentors or have their children’s back in another way. They are another part of the social network a protagonist has and can be very useful that way.
I have a few characters who are orphans myself (Jane most of all, while Inez is actually having her adventures with her adopted father), but then, I’m not dealing in teen or young adult literature and all of my regulars are well above eighteen and thus adults by the law. My reason for Jane being an orphan (like everyone else in her department) wasn’t to keep parents out of the equitation, but as an explanation how she could start training at the age of ten and how her colleagues can easily change identity when necessary - they all have no family outside the agency who might worry or whom they might miss. On the other hand, I also have mothers in my stories. Alex Dorsey is very close with her mother, who provides emotional support and always has. John Stanton’s mother is regularly mentioned together with the rest of his family. Isadora Goode is estranged from her mother as much as from the rest of her family, while Lisabet Lewis’ mother still worries for her, even after she’s changed sides.
Mothers can be very useful. They’re another source of information for your cosy detective (since mothers often know all and sundry and can easily fish for information). They can provide useful objects (from an old piece of jewellery that has been in the family for generations to just a hamper with food for the nightly stakeout). They can simply be sounding boards for the main character, listening to them and providing another insight. They don’t deserve death, they’re far too useful alive.