I would assume because the guidelines for the communications platform being used do not require the use of their actual face for their profile pic, and they have selected a picture or character they like.
I was pondering too, and I assume, (not necessarily se do that)
1) Cute, it's analogous to cats, people love cute thing and show them everyone else
2) They want to be perceived cute. Hey look i'm not seeking conflicts, i'm just a little cute thing hehe.
3) Real world is boring, lemme frame the perceived image of myself into the world i like
4) Childhood was more interesting anf full of adventure, no responsibilities
5) Colorful things more stimulating and appear richer to the eye
6) Seeking for socialization and for communication with like minded people: hey if you like the pic, then you're in my circle already
Not true. Smart software engineer doesnt set anime pic as their pofile pic
I set my profile pic at work as two hamburgers. People set what they like.
Because there weren't any good American TV shows in the 2000s.