You asked who drinks within those limits.
Quite a few people, you just don't know them yet. My partner a previous heavy drinker, had 2 units of alcohol last night = 1 pint of beer we were out at a party where there was a lot of alcohol flowing. He didn't have any alcohol before or after. Ok that is the influence of hanging out with me, but we were still giggling, smiling, relaxed, loving, dancing, socialising. Neither of us consume any sort of drug or smoke tobacco ( he used to smoke, I have used recreational drugs a long time ago ). It has taken me a few years to get to this utterly relaxed in sobriety point but I'm trying to say, any drug is not a requirement for a fun time.
I made a post a while back uncovering government 'limits', they aren't based on much evidence and are in place to recommend a 'limit at which there is not much significant increased risk', however from the few things I've read any alcohol is worse than being abstenant - it's a poison.

Problem is everyone thinks these guidelines are some sort of target, or acceptable. Even I went for the target of drinking 'within guidelines' when actively using TSM.
We've become used to alcohol being a normal part of a human's life, yet it's quite damaging even in small quantities.
This isn't coming from a holier than thou state, and I don't expect anyone here to go completely sober. It does however come from a change in viewpoint and seeing things from a different angle as well as doing a bit of reading between the lines.