Because so many of us have grown up around it, with parents who overdo it on the booze on the regular, and find the culture surrounding drinking to be incredibly weird. If you can't have fun without drinking, you're probably an incredibly boring person or so full of insecurities you don't know how to let loose without substances.
Additionally, I know the older generation didn't have their every move papped and put online - so getting drunk probably felt safer. Now, younger people are more image conscious because they're much more likely to be in the background of someones photo - and if you're too drunk, you're probably going to get caught in a bad way. (I know this bit is very ironic when I mention alcohol and insecurities in previous paragraph - I think insecurities manifest different between generations tbh)
It's literally a poison, it has no real health benefits, and we are more than aware that the health system is is falling to parts so we are trying to look after our health a bit more. Links in with the rise of run clubs and popularity of gyms with younger people and things like hyrox etc. Again, probably many of us have seen our parents health fall to pieces through excessive drinking and want to preserve us for longer.
The cost of it - I live in a big city centre, its £6 for a pint or £9 for a glass of wine. Many of us just do not earn enough money to regularly pay that out - the friends of mine who do drink are significantly worse off financially than those that do not drink (but still regularly socialise) - despite earning similar amounts.
I'm not anti-alcohol btw, I have the odd drink here and there (it flares up my endometriosis so I don't drink much tho), but these are the main thought processes I here when talking to my friends and peers about this stuff (I'm late 20s, but friends from early 20s to mid-30s).