Why do you need to use pixel art for 2D games?

Hey, I'm planning to create a 2D game but I was wondering why do most of them use Pixel art and not regular drawings that you would do on photoshop or something, by photoshop drawings I mean like taking your brush tool, draw a sharp clean lined circle, color it with 32 bit shades, delete the background for the transparency, and just use that for a moving picture in your game.Why do you need to use pixel art for 2D games?
It feels retro because most games in the beginning were 2d and pixel art, there was no "regular" drawings back then.

It more like a tribute to these old games.

portuguese translator