why, because it's pretty?

It is, in fact, pretty, appealing to the senses and possibly emotions. But it's not just the aesthetics that make it worthy of consideration as art.

The gameplay itself is creative; the player can solve levels in unintended ways. There are (sometimes surprising) interactions between various elements, and it is their arrangement that the game is based upon. The layout of various levels is subtly expressive; there's a sense in which the player is getting inside the level designer's head by playing their level (this is, IMO, an overlooked aspect of many video games. Level design is an expression of the designer's thoughts!)

There are some definitions of "art" this would not meet, and others that it would. At the very least, then, it's worthy of consideration.