Early-stage startup hiring hinges on delayed gratification. People are always giving something up - easier hours, better pay, stability - in exchange of future potential value.
Maybe that's at the root of the ping-pong tables, foosball and free beer. They give a modicum of immediate gratification, keep the monkey brain quiet.
@Tak True. Every extra hour spent with a beer, on the couch, in close proximity to your laptop, also increases the chance that you'll just go back and do some more work. Chat at the pub may deviate from whatever you were working on, you may run into other people, noise will make meaningful work conversations difficult.