What exactly are the parameters of looking 16, 25 or 38?
In enough detail to stake your job on? Or possibly a large fine or prison sentence (obvs for alcohol but it has been mentioned here) - get it wrong and you get disciplined, sacked or prosecuted. And if someone is ill, injured, or does something sad or stupid after you've made this sale, you'll get the blame for that too. And if the premises lose their alcohol licence, or have restrictions imposed on it as a result of your actions, and fines issued, then they're your fault too, any revenue and jobs lost because of it, your fault.
And you get a whole couple of hours training, probably online, to equip you for that responsibility.
A teenager local to this area died recently as a result of alcohol. There's practically a witch hunt for the retailer of the alcohol, and legally as the teen was under 18 (but did look older) they are also responsible, even if it was a proxy sale and they suspected (or more correctly the law decides they should have suspected) that was the case.
Probably the same people who'd be screeching that you can give your child booze at home at 5 years old or that a 16/7 year old can drink with a meal whenever the issue of ID is brought up.
That's why. I can't imagine how I'd feel right now if I'd given that young person access to alcohol that day, far, far worse than being balled out by an entitled knob head because I ask for ID because I'm not sure. And that's before any legal ramifications that might happen.
I mean we can always campaign to have things changed and to not hold retailers responsible when things go tits up and people are hurt as a result of the sale of age restricted items, and people be responsible for their own behaviour.