I feel like fan fiction has given me unrealistic expectations of character development because after months of reading only fanfic, I opened an actual YA novel and boy was I disappointed
If you wish to make an apple pie from scratch, you must first invent the universe.
Any given fanfic writer is walking into a fully-stocked kitchen. The characters exist; their milieu exists; their backstories and traumas and interactions exist. We come in and start cooking, adjusting recipes to our own tastes–and most of the time we do that while sharing the kitchen with hundreds of others so we can see what they’re doing, adjust our recipes and techniques based on what we like or don’t like of what they’re doing–and whoever eats that food is probably eating it after eating dozens or hundreds of other dishes from the same kitchen, which inevitably color their opinion of the dish in front of them.
Original writers are working alone. In a void. They don’t just have to buy or grow the ingredients; they have to build the kitchen.
There’s absolutely nothing wrong with preferring to hang out in, and eat the food from, a crowded kitchen whose foundations were laid before you were born (which, for all of us in, say, comics-movie fandom, they were). But you can’t expect the same experience when you walk up and hold out your bowl to someone cooking over an open fire in a lean-to on the edge of existence.