Speaking as someone who until recently was permenantly skint and has now got to 'not quite skint' there's a reason for this.

I would always pay my share and I would never accept a round of drinks except when it was blatantly clear it wasn't expected I'd get one in because when I went out I had a strict amount to spend.

I didn't want to be seen as scrounging or mooching off my richer friends who really had very little understanding of what it's like to be skint. If someone offers to pay for pizza one week, great - but that means at some point in the near future I have got to find 40 quid or whatever. Even if I am not paying for the previous few weeks, if I'm really skint I just can't put that money aside because saving that kind of money when you are skint just isn't possible.

Also if everyone pays their share, I can get cheaper options, plain pizza or a pint of something cheap. That means I can spend less and still enjoy a night out.

If you've got a group of people with significantly different incomes, everyone paying their own way just works better.