Because that's how language works. vtuber/pngtuber describe a specific form of avatar, and people aren't gonna say 'they're a streamer that uses a 2D reactively animated animator' when they can say 'they're a pngtuber' and be perfectly well understood by the audience that is actually going to watch them instead of telling them to get off their lawn.
I may be using the terms incorrect, but in my understanding the pngtuber/vtuber is "the person doing the streaming/video", while the "avatar" refers to their representation in video. So saying "pngtuber is different term for avatar" would be like saying "person is a old term for selfi" right?
This discussion is complicated by the fact that a vtuber in character is identical with his avatar, so he will refer to his avatar in first person, e.g. "My camera broke, I'll be a png-tuber today."
The {png,v}tuber is the character. For many such streamers there's relatively little difference, but some of the bigger vtubers are actually a team of people who may take turns playing the character. (Think like Hatsune Miku)
