As a european, I didn't knew this was not a thing at McDonalds. I have always seen beer served there and I don't know why it should be exempt from serving it.
During my long-ago employment at McDonalds, nearly all the staff was under 18, and so ineligible to be handling alcoholic beverages, even 3.2 beer. Many American states are quite fussy about this sort of thing--I've had a younger cashier at a grocery store call over an older to scan my carton of beer.
Now I guess there are more of the over-18 or over-21 employed at McDonalds, but it might disrupt the flow of ordering if all orders with beer had to go through particular registers.
In Germany we're allowed to drink beer at 16 (everything at 18) and afaik if you, aged 15 to 17, are working somewhere that sells alcohol there isn't a law that's forbidding you to handle it.
I am not a regular customer but I don't think I have seen that many under 18 working at McDonalds tbh and the few that would could be asked to just prepare the burgers and stuff or handling payments.
In most european countries, the McD menu has the local table booze (beer, wine, …).
In several countries McD actually has its own McBeer (might be the sole offering, or be alongside the country’s usual) although I have no idea what goes into it or how good it is.
https://youtu.be/6Pkq_eBHXJ4