I mean, in this particular story's case it's because she was too special/OP in her past life and fetishized being normal -- and just utterly failed at living her dream because God is apparently a filthy kinkshamer.
IIRC she assumed she was reincarnating into a world like ours, where being an 'average human' (or more accurately, an 'average first world Japanese human' because that is what she was
really thinking of when she said 'average') is actually a pretty good life and is significantly better than like 90% of the rest of the human population in the world.
The god tried to explain to her that she was in fact reincarnating into a medieval high fantasy world where humans were on the bottom of the food chain and the 'average human' died early and died hard, typically because they starved to death, were eaten by monsters or caught some kind of disease, but she was too busy wallowing in that Japanese cultural fetish for 'being normal' and 'not standing out' to pay attention to the god's perfectly reasonable explanation about how she had no fucking clue what she was asking for. The god was bound to her request though, and since he was actually doing her a favor and didn't want to send her into a life of suffering and pain just because of her own stupids, he decided to take an
extremely liberal definition of the term 'average' to be the mean of
every living creature instead of the intended 'average
human' that the protagonist had wanted, so she ended up with the magical and physical ability of the average between the weakest living creature in the new world and the strongest, which turned out to be around half as powerful as an ancient dragon or 3600 times as powerful as the maximum a human was capable of. Which promptly triggered her prior trauma about being special and led to her trying to pretend really,
really hard that she was normal, and failing entirely to do so in progressively more exaggerated ways.
All of which could have been avoided if she had just listened to the god's explanation about the world she was reincarnating into instead of blindly assuming that it was exactly like the last one like the spoiled, arrogant brat that she was.
I mean, I dunno about you guys, but if I find myself before a godlike entity offering me a chance at reincarnation, I think I would be inclined to listen to anything it has to say about the nature of said reincarnation, or anything really, because it is a godlike entity and I am
not.