Why? Because I can!
Seriously, some shoot digital, some shoot old. bulky ULF cameras, some shoot instamatic, some shoot polaroid, some shoot p&d 35mms and some shoot the latest and greatest analog monsters. When I'm out shooting an event nothing replaces my EOS 3/1V bodies. Sometimes you need the speed of the AF, the reliability, the ability to use the latest and greatest lenses and sometimes the hefty, bulky look of a "serious" SLR like a 1V with battery pack lets people know you mean serious business. Bad for street photography, great when moving people out of the way when you're officially photographing an event of sorts. And then there's stuff like advanced flash techniques which won't work with old manual cameras. Whenever I need a flash I really like a camera-/flash combination I can rely on 100%. Oh, and you mentioned burst rates. Well, I can get an EOS 1V for very little money and can shoot 10fps if need arises. Good luck finding a digital camera with the same burst rate at a similar price level (even when looking at the price tag these carried when they were new).
Different jobs need different tools. There's a use for the latest and greatest as well as for something like my MF-Minoltas when I'm out going for a walk and don't want to lug all my gear around.