Why don't they do a realistic graphics version of Minecraft?

That is the entire purpose of Minecraft.

If you can't get past the graphics of Minecraft, then don't play it, or install mods if you have to have high-res textures. Minecraft has never been about the graphics, and truth be told, graphics aren't the defining feature of the game anyway.

Probably for #1 Retroness #2 Easier to texture #3 Because they were inspired by a different game. Minecraft stole the whole blocks thing from an old gem called infiniminer.

They have sims for this reson i'm pretty sure…

For several reasons.
1. It means the game is lightweight and can be enjoyed by people even with low emd phones or computers.
2. It appeals to more people. Not everyone wants to be playing a realistic game that looks like you are actually killing things.
3. It suits the game. The idea is that everything is made of blocks. This means even the blocks are made of tiny blocks (the pixels).

If you don't like the style you can always download a graphics mod for PC.