Holders are readily available or free. Kegs are extremely hard to transport. Whether it is in the car, shipping via a carrier, or just outside on the porch. Indeed, disabled persons or people with only a moderate amount of upper-body strength may not be able to transport full kegs or C02 bottles at all. Beer bottles pack up nicely into easily handled packages, whether they are six-pack carriers, boxes or the bottles themselves.
Bottles are always ready to drink with no extra work.
No one ever grabbed a bottle and wondered if they had the proper equipment and gasses necessary to access the beer inside. If one of your friends at a party tugged on your shirt sleeve and asked how to get the beer out of a bottle, that would probably be their last beer for a while. Bottled-conditioned beer tastes better. Some of the best beer I've tasted was filled from a keg to a bottle and let sit for several months and underwent accidental bottle conditioning. The same beer from the keg didn't taste as good.