Because "keeping it reasonable" doesn't scale so BigCos frequently dis-allow drinking during/immediately prior to work as a blanket policy in order to have grounds for firing anyone who does it to excess.

Also drinking on lunch break is unprofessional (even if you don't do it to excess) in a lot of blue collar fields. How many white collar people lamenting the lack of beer at lunch would turn around and lose their shit if their crane operator had a couple beers? That's a double standard no workplace needs.