If no one points out the holes in your work, you don't improve, your ego inflates, and it's very likely you might delude yourself in the future (and turn out like EL James).
The massive error here is to assume that you, as a reader, have unique insight into the method with which the writer is planning their improvement (maybe they already are working with a beta-reader or editor that they trust. You are just a random person). And even whether this is a phase in their life during which they have time, mental energy and desire to work on improving.
There's a tendency to think, as a reader, "I spotted this flaw! I should let them know, so that they can improve! They should be grateful that I'm helping them out!" But consider some of the following:
Maybe your'e not the target audience. My fics are full of angst; non-fanfic readers will immediately notice long drawn-out emotional angst scenes and will complain about that. My target audience, though, loves that stuff.
Maybe your comments conflict wildly with other comments! I actually find comment sections to be practically useless for a source of critique, because every reader has a different take on things. For example I got these comments on my last fic: "Your Castiel is spot on and so in-characer", "Your Castiel is a out of character", "This fic is paced way too slow", "I really, really love the slow build". What on earth can I do with that?
Maybe they already have a chosen beta-reader or editor or even a teacher. My resolution to the above problem of wildly conflicting reader opinions is that I seek out particular people whose literary judgment I know and trust to give me feedback. As further background, I am also a musician and I've learned over time to pay no attention to critiques from random people, because I simply don't know if I can trust their judgment. They're random strnagers. I know nothing about them. Instead what I do to improve is seek out the very best people - the best musicians, the people with the best ear - and pay them to be my teacher. Those are the people I pay attention to. I ignore everybody else. I've done the same with dance, horseback riding, skiing, and now with writing. When I want improvement I go to the best expert I can find and work with them one-on-one and I open myself to them fully and try everything they suggeset. But I actually DON'T any longer pay attention to random critiques from total strangers.
Maybe they're in a phase of their life where they're just writing to get a mass of words on a page and are not in a self-improvement trajectory at the moment. Again using me as an example, I'm planning to work on improvement my writing technique later this summer, but this spring I'm just trying to finish up some stories that I've promised to my readers.
Maybe they could have made it better if they'd just had more friggin TIME! This is a huge one for me!! Fanfic writers are writer who are squeezing it on part-time around another job. I often have to post chapters in a huge, huge hurry squeezed around an extremely busy job (which I am now late for, lol) because I promised my readers updates on a certain day and I have a thing about not missing update deadlines. I often know exactly what the flaws in the work are - sometimes I'll even warn my readers "it's 3am, this chapter has flaws XYZ but I'm out of time so here it is".