You also must have all of the C02 equipment on hand anyway to seal the keg initially and dispense the beer later, so there's just not much point to priming. New and nice gleaming, shiny kegs are readily available, for a hefty price. Bottling encourages better sanitation. With no complex, hidden surfaces that contact the beer, there is less incentive to skimp on sanitation. Finally, kegs have a bit of social stigma.
Kegs carry a connotation of excessiveness.
I started out bottling like most brewers. I now bottle and keg, depending on the beer and what I want to do with it. I have several good reasons that are unique to where and how I brew. First, I already had all of the equipment purchased before I realized most of the pitfalls of kegging, so the financial damage was basically done. Next, before I started brewing, I already owned a 1,500 PSI power sprayer, which makes short work of keg cleaning.