Because the block that generated the coins might get orphaned - e.g. due to another longer chain - and the coins would have never existed in the first place. AFAIK it was 120 but has been reduced to 100 blocks.

The 6 confirmations rule is arbitrary, I am fine with a single confirmation for low value TX. The more confirmations the higher the probability that the blocks containing the TX wont be orphaned and the lower the probability that a doublespeed attack is successfull.