Actually a very good question.
TLDR: Because they feel emasculated.
A beard is a complex social signal: a man feeling powerless, as if he has no future, wishing to signal virility but closed off from proving himself by the hegemonic culture, no rites of passage, it’s a sign of Letting Oneself Go, and from this, rebellion against capitalist norms (cleanliness required to work with machines safely, get hired at an interview being well kempt and not being shabby, self-respect), a sign of testosterone in an age of estrogenic influences, a traditional marker of solidarity in the working class (shaving instruments were expensive and a Good Shave a sign of wealth), mild competition and aesthetic expression regarding type of beard etc etc.
Rich men don’t need beards, at most they’ll have stubble from travelling but be embarrassed of it. They find them too grubby, in my experience (do any of the Royals have one)? It’s viewed, here, as a Middle Class Scourge, because they aren’t imitating anyone in particular. It’s an image. The urban social climbers think it’s authentic and rustic and very Countryfile.
This impression carries over to business. Bearded men are seen as deceitful, note the stereotypes about types of beard and various character flaws. It can obscure or alter identity, make you hard to really see, who can trust that?
They are hiding something: their insecurity.
Men are being outcompeted by women at school, at university, in academia, in the workplace, in Government and so forth, what’s one visible thing men can do with their appearance, that women cannot take from them (as they, silly, see it)?
With increasing androgyny, this is like, the ONE thing, they’ve got. Same problem with lad culture (really Eurotrash Ibiza club culture from the 90s, spare us), which has mistaken physical maturity (fucking, drinking, drugs, swearing/being rude to all women as a form of underclass ‘humour’) for social or intellectual maturity (and respect).
A clean-shaven man doesn’t need to look like a furry animal to show dominance and a masculine nature, he has the real-world power and adult behavioural abilities in reserve that requires no display or show of symbols. In fact, counter signalling is their game.
It’s like the guy in credit card debt who buys a sportscar versus the millionaire who buys a secondhand moderately nice car on equity, because who does he need to impress?
The blokes who say women are attracted to status? Ever notice they don’t have any to speak of?