Why are bars the social default?

They're so expensive. Jon and I were barely able to pay off my credit card balance in full by the due date thanks to a little bit of Google stock that we sold but once we pay rent on Monday we have about $10 in the bank and $20 in cash (not counting our coins) until I get paid in two weeks (or until he gets paid, whichever is sooner). So, I'm not exactly inclined to throw my money away at a bar on Friday night when we have so precious little right now. I invited people over to our house to hang out and drink here instead since it's a lot cheaper to buy some beer at the store but I don't think they're coming :-( It's a bummer to have to decline a social invitation because you can't afford it. Oh well. We'll have more money soon and in the meantime we'll keep being frugal and declining invitations to bars.