Ever since Atsumu was young, he knew that life was unfair, people weren't created equally, they weren't presented equally, he knew that no matter how much people protest no matter how much they stud up for the good cause. Some people just don't change.
Living in a world that revolved around power and success, the most important thing that people care about were their genders, their second genders to be exact. Alpha's were at the top of the food chain. Presented as an Alpha and one was guarantee success.
Same with Beta's. But being an Omega, that's what got people to grimace, Omega naturally presumed weak and at the bottom of the food chain. They were a small minority in the world, society trended to forget that, above everything else they were human being as well.
Atsumu though that his life was pretty good until he presented as an Omega.