Let me know if I’m spewing gibberish, but sporting events go back a long time, to a time where there was only radio (no television). The radio commentators set the standard (no dead air, describing everything to the blind). Which, I assume, was copied over directly to television. Fifty years later, commentators still start out by working on radio and hope to get that promotion to television. I understand, that a lot of your sporting commentators in the US come from the sport itself, maybe those that don’t have any experience might conform and do what’s expected of them by the standard their colleagues have set before them. So blame the standard.
That’s not to say all commentary is bad. I have the option to watch Formula 1 on three stations (Dutch television-> one guy, with commercials, Belgian & English television-> multiple guys, no commercials). I prefer to watch it on Dutch television with commercials, not because it’s in my own language, but because the guy knows what he’s talking about, has insider information and often tells his audience what’s happening before the Formula 1 television director (who has access to all angles on playback) knows. But more importantly he doesn’t have to compete with others to keep his job safe. As soon as multiple people start yapping it starts to become a competition for airtime and attention. Without a good commentator Formula 1 would be boring (in fact, when I went to a live Formula 1 race, I used a radio to listen to the Dutch commenter’s commentary).
What amazes me though is the Tour de France, an etappe (day race) takes hours (during which nothing spectacular happens, till the last 2 minutes) and there is not one second of silence between the commentators. Might have something to do with the fact that people start zapping when they hit silence.
Edit: I could have just removed all my incoherent babble and leave the three sentences I highlighted, however in the spirit of the question I decided to leave it as is :)