Why
are all the planets round?
A good question. Are they, though? How many planets do you know of?
Well, last I checked there were 8 planets in our solar system. And yes, they're all round (if this is the word you choose to use, because they are far from spherical).
And the simple reason for them being round, is gravity. The same way you and I are kept on the surface of the Earth, all the rest of the Earth is as well. If the Earth would be a lot different from a sphere, bits on the furthest ends from the center would either fly off (if they're far enough) or come crumbling down to earth, just as would you if you went as far as that from the center (and tried to 'stand still' as you do on the current surface).
As for the big bang having no proof, while I'm no expert, I'm pretty sure there are things that suggest (if not prove) that the big bang really did take place. One that pops to mind is the background radiation (the static on your old TV if you have no cable/antenna connected), but I'm sure it's not the only one.
As for asteroids 'not hitting any others', I don't know if you've ever seen a picture of the Moon, because if you had, you'd see that quite many have landed there. Granted, they probably wouldn't have contained too much water (otherwise we'd probably have 'found' it by now). In fact most of the asteroids in the solar system get sucked in by the far superior mass of Jupiter, than Earth.
Whether water came to earth on an asteroid, I don't know, but saying that asteroids don't hit any other celestial bodies, is kind of stupid.
I don't know what kind of footprints you're talking about, but evidence (to my knowledge) does suggest that mankind wasn't evolved when the dinosaurs ruled the Earth. Carbon dating is a pretty accurate way to date remains and (even though I haven't checked any of this per se) all human remains dated are younger than any of the dinosaurs' remains.