Ok so it seems obvious. A wish fulfillment story is a story in which the characters are trying to fulfill some sort of major goal or wish, but the thing that confuses me about this genre is- what story out there are the main characters not trying to fulfill their goals?
I mean, if there is some story where the mc has literally no goal at all and is just blindly blundering around, would this story even be worth reading? There's gotto be something I'm missing here.
Is it the condition of self insert? Is it that the wish has to be something that the author wants to achieve but cannot because they live in real life, therefore they do it through their story?
Trying to fulfill the characters goal isn't what wish fulfillment is. Wish fulfillment is trying to fulfill the READER'S/AUTHOR'S dream goal.
To explain it in context of content you may have read, in japan they deal with a strict society where if you get sick, you use up vacation days before you use up vacation days before sick days. In that sense, many Japanese want to escape the stressful society and being isekai'd is like their vacation. From then they go on to try all kinds of foods (popular in japan) and build a harem "against their will" (also their dream but they can't actually say it publicly so they pretend that the world forces them into the harem and that slave)
In China, they don't have much movement on the social structure and pretty much the government has full control of them while those who are related to those in power reign. Their wish fulfillment is to crush all these young masters and get out of their oppression and oppress others themselves by taking over their position. They also after all those jade beauties. Thus you have the whole xianxia.
In Korea, the nobles of the time pretty much turned into corporations. (similar to japan), but since the change was much quicker, society has become more materialistic. But there isn't much class movement as corporations own everything. Thus many KR novels center around MC making a ton of money. Then forming a friendly partnership with some billionaire corporation.
Effectively, wish fulfillment novels are ESCAPISM of achieving your dream through a novel. But since many don't want to put effort into it, the plot pretty much spoon feeds you into your dream.