Because some people are willing to pay that much and they're charging build labor.
Honestly, shops need to start marking down their wheelsets to stay competitive with online. I was quoted at around $350 for a 7600 laced to a deep-v with HALO coating. For just the rear.
QFT.
Guy I was talking to quoted me a build with Phil Hubs, said it would run $650 and not a second later says, ****, I can't charge that for a set of wheels...Hmm.