1. If, when you rammed another aircraft from behind, that aircraft took damage, then it would introduce unreal levels of sillyness. For a real combat pilot, near misses can be scarier than tracers appearing over the canopy. After all, your airframe can conceivably survive a few holes. Not so with a mid-air. But this does not apply to computer sims. Overshoots would be much more difficult to generate against people content with M.A.D. if they rammed you, half the players would go all kamikaze on buffs, etc.
2. This has been explained to death. Due to net lag, everyone is flying in their own slightly different paralell universes. If you ram another plane after, and it says "You have collided", say during a HO pass, that means YOU have hit his airplane in the world your computer screen is showing you. If it doesn't say "XXXX has collided with you", that means on his screen, in his paralell universe, he passed you by clean.
