My mom or relatives always made mine as a kid, and then I made my own when I got old enough. (By "made my own," I mean I bought some pieces and then made some other things out of paper, cardboard, etc. -- not that I sewed elaborate things.) For me, it seems like tradition to make a costume, and I really don't like the idea of buying one for my kids.
I've always made my older kid's costumes (except when he was a baby, and we just got him an orange onesie). My younger kid is now still a baby, and we just got him a onesie. I was a little disappointed when my older kid just wanted to be a ghost this year and cut holes in a sheet

Seemed to simple!
Anyway, I never judge anyone for buying costumes, though. I just think it's a fun chance to be creative. I also like the idea of not "mass produced" costumes, so I try to steer my kid away from "characters" to other things. I feel like it's more fun to be creative and come up with your own thing. But I totally get that other people don't want to do this. Who cares? People shouldn't judge each other so much!!