Furball,
Why don't car's constantly collide on interstate?
Same reason, they are all basicly going the same direction, with similar speeds.
Even if a pair of bullets did "kiss" they'd impart pretty small angular velocity as a result.
Chances of a fuzed shell hitting another bullet hard enough to set off the fuze would have been pretty remote IMO.
Thats not to say it never happened.
But if your watching a swarm of .50 rounds going downrange, how would you know if 2 of them touched?