Why are so many people insistent on misgendering characters?

Currently applies mostly to Kris, but also to Frisk and Chara- Kris is just the place I'm seeing this the most currently (also I know this is the UT subreddit, but it's just getting to be such an issue I feel like it needs to be brought up and unfortunately I don't post enough on Reddit to say anything on the DR subreddit). I've seen so many people justify it with "well it's my au I can do what I want," but I haven't seen this same scale of misgendering for any other characters. There's the occasional genderswapped AU, of course, but you don't see a tenth of all fanfiction about Mettaton referring to him by she/her pronouns, or switching Undyne to be a man when shipping her with Alphys, for some basic examples (not saying that doesn't happen, just that it's not currently happening at the same scale right now). And I can't help but notice how most of these people will default to he/him for Kris especially. I know blah blah blah you're making a big deal out of nothing, but seriously, the casual ways people will just interpret a non-binary character as amab and then proceed to vehemently ignore their canon gender identity is really worrying. It's not just a thing that's going on in the UT/DR fandom, after all. So maybe I guess my question isn't why is this happening, but how can it be avoided? As a non-binary person, it really hurts to see a fandom space I love show so much disregard for my gender identity, and even though the community is very welcoming, of course, it's still painful. Is there any way of changing this?