Why are some self inserts so cringe?

I watched the first two episodes of I Got a Cheat Skill in Another World and Became Unrivaled in The Real World, Too which of course telegraphed the cringiness from the name itself but I wanted to check once. Other than some hilariously bad production value(they seriously used a cutout of 3 characters for a whole scene where their faces didn't match the situation at all but still they had to include those3 supporting characters in that scene IDK why it just looked terrible) the lack of self awareness is so bad. So a fattie intervenes to save a beautiful girl who's being harassed in public in Japan, gets beaten up then finds a hidden away door that can be used to travel to a parallel word and has a spiderman style genetic change that turns him into a /fit/ handsome man with six packs. He sees these six packs and doesn't try to check what he looks like once till he goes to school where his old bullies are obviously at his appearance and all the grills are miring. At this point the hot girl he saved comes and offers him a position at the super elite school her father runs and even here he's so handsome that all the students keep staring at him during lunchtime. So then later when he asks the heroine how she recognized him despite him losing like half his BMI since he had saved her she said it was because he had the same 'kind eyes'. I was watching to see how trashy it could get but