Because playing piano is extremely fun and rewarding, and offers cognitive, emotional, and social benefits. The piano has a huge range of sound, from low to high, soft to loud. Because of this and the ability to play multiple notes at once, learning piano is the best way to learn to hear music and understand music theory before picking up other instruments.
There is a huge amount of research to support the benefits of making music at the piano. Here are a few articles about this: