It's a probability thing.  THe probability of it happening is so low as to be negligible.

Even if you could generate a trillion addresses every second for the rest of your life, generating someone else's address is still less probable than the chance that Barack Obama will pick you up as a hitchhiker, take you to a gas station, where you'll buy the winning lottery ticket for Mega Millions at 12:12pm on 12/12/12 with a ten-dollar bill that fell out of an overhead helicopter into your shirt pocket while the Pope is simultaneously being struck by lightning.