Thanks for the question!

There is no rule stating that you can't play in the middle square. The middle square acts as a wild card unitl someone plays a piece on it (it counts as every color, shape, and number at once). Having it empty allows players to more easily make higher scoring plays.

Additionally, at times there can be a concept of controlling the center as no one can place a tile where you've just placed. So if you control the center, they can only get three in a rows on the side.

Hope this helps!