In most places it's nothing to do with tips (why do Americans bang on about tips the whole time?), it's more a matter of social custom, which I think is what the OP was asking. And, in many places, with increasingly stringent drink driving laws, bars are much more used to customers not drinking. But clearly you have to buy enough food or drink to earn your seat for the evening.

Certainly, in the UK, pubs are only surviving because of food nowadays.