Why do so many little kids play minecraft?

Because it's a game meant for children to enjoy?

They like building stuff.

However, I find the square tiles annoying.

Not only little kids, grown teenagers too,
its a game like many others that got popular for beeing different, as a sandbox game it really lets you build anything you want, the blocks and stuff its like a signature, most may don't understand the fun of it, but once you create a massive battle tower with nether world portals, reach level 50 or higher in xp and defeat the ender dragon using any enchanted weapons you created killing monsters, then you may find the joy of it.

You are implying something. Do not stated as fact if you can't back that up. I'm actually surprised if this is true since most of the kids just play call of duty and Battlefield.

Bc they like it.

Because their parents find it acceptable for them to play in a open sandbox game. Its fun for everyone and its not violent like almost every game. Not a hard concept

It lets them be creative and it can be simple for kids to play. Put it on creative or peaceful for them and they'll love it.