Because humans are destined to colonize space, and this is just an early step in a journey that will take hundreds or thousands of years.

More importantly, challenges like space exploration help drive knowledge and our economy; and are critical for national prestigue.

(And, most people don't focus on this, space exploration is a way for the US to demonstrate its military technology in a non-antagonistic way. There's a lot of overlap in space exploration technology and miliary technology.)