It's not really a big deal if someone sneaks on board, therefore, no reason to burden millions of flyers every day with additional checks. Also, you may well not have ID on you. Maybe you don't like to carry it. Maybe you forgot your wallet. Maybe you don't even have one. Who cares?
I've taken many flights in Europe without id check, see also: https://travel.stackexchange.com/questions/180781/what-deter...
It's not really a big deal if someone sneaks on board, therefore, no reason to burden millions of flyers every day with additional checks. Also, you may well not have ID on you. Maybe you don't like to carry it. Maybe you forgot your wallet. Maybe you don't even have one. Who cares?