Or maybe one of those less-optimal characters catches on and goes somewhere. I've seen this both as DM and with a few of my own as player, and it's great when it happens.
I've also found in my games - and have the numbers to back it up - that starting stats really don't have much if any effect on a character's eventual career length, measured in adventures. What makes a much bigger difference is making it to your third adventure, regardless of your stats, because the numbers show that if you get to your third the odds of you having a long career jump dramatically.
My educated guess as to why this is, not supported by any hard numbers, is that by the time a character's got to its third adventure three things have happened:
1 - it's acquired some useful gear and magic that helps it survive
2 - it's gained a level or two, which helps it survive
3 - it's become an integral member of the party, thus is more likely to be revived (and thus be able to continue its career) if it dies.