For the same reason men climb mountains, columbus sailed the seas, Lewis and Clark headed for the unknown west, we went to the moon...
So the reasons are...
1. because it’s there
2. because I can
3. because I want to
4. and who knows what I’m going to find there
Actually Musk’s stated motivation is to provide another base for humanity, a redundancy if you will, in case something horrific were to happen to our earth and wipe out humanity forever. A rather noble, chivalrous thought, even if a bit Quixotic.
Mars has no breathable atmosphere, hence we will always be in a relatively fragile shelter. It is no “Plan B” for Earth.
The analogy I come up with for Elon Musk & the guy at Virgin Galactic is Sir Walter Raleigh. The Roanoke expeditions to America are a useful comparison. That one failed, but others succeeded.