I actually live near the G-burg battlefield and have seen the fused Minie balls myself. The difference is that these collissions resulted from massed volleys of sub-sonic rounds fired directly at each other at relatively short range, thereby increasing the odds of collision. Satellites, on the other had, are pretty much solitary bodies, traveling in singluar orbits. The odds against an accidental collision are much, much greater.
Box battery flak like the Germans used in WWII was also relatively ineffective but tell that to the unlucky ones.