Some of it's cultural, like you say.
Some of it is company concerns about public perception since older generations of Americans (who are the biggest revenue demographic) still have some negative associations with beards, at least that they don't expect authority figures to have them. Overseas cops, military officers, doctors, etc commonly have beards/goatees.
Also companies frankly don't want to be in the business of adjudicating every individual pilot's interpretation of facial hair SOP, and you know that would be a problem.
Along those lines there's still the safety issue if it's a little too long...