It's not that LA cannot or does not have any good bus service at all, it's just that the MTA is playing catch up. It wasn't that long ago that locals and their representatives were reflexively anti-transit.
LA has a fantastic 24-hour BRT line in the Valley, the
G Line, which runs in an exclusive dedicated ROW from Chatsworth to the North Hollywood B Line subway station. I've seen some of the G Line stations and crossed the ROW many times since I moved back, and it is impressive. There is another BRT line, the
J Line, from San Pedro through downtown and out to El Monte. I know that much of the line is grade-separated, but I'm not sure about the split as I've never seen it myself and the Wikipedia entry doesn't really say.