Why can't we grow beards?

For me mostly genetics.

Because we volunteered for the military, and they dont let you! Next question?

beards cause many hygene problems when in the field

I know that in many services, facial hair is not allowed because you cannot get a complete seal on an SCBA or a mask (chemical attack, etc...). Your beard would not allow for a proper seal with the mask and that could put your life and others at risk.

It is not conducive to the environments in which Army personnel need to operate, with the exception of those personnel operating in areas where beards are an integral part of the society and considered to be a sign of manhood and someone to be respected, there really is not purpose for a beard.
They are not compatible with gas masks, can become an issue when not maintained properly causing many different problems to include lice infestation. Furthermore beards have never been a part of Army history except for the earlier years when beards were more of a socially accepted dress for men.
Suck it up, grow a pair and get back to being a soldier, I you want to grow a beard, go Special Ops and live in the desert, it's not really fun. Or, get out and grow one of the many hipster beard fashions.

Fair enough-beards are fine if it doesn’t interfere with a specific utilitarian purpose like wearing a pro-mask. I don’t support the culture of conformity. A free country should have a free military.

Over here on post many, and I do mean many of the black guys have a beard. On the posts I've been to, the black guys get to have beards and white guys are told to bleed and bear it.
Also, the Navy had beards back in the 70's I was told.
Finally, the leftists interested only in equity in our ranks would call beards sexist because the females (mostly) couldn't grow them. I think it should depend on MOS.

Understood and appreciated!

Can't get a good seal on gas masks and SCABs with a beard

All the branches opened this Pandoras Box by allowing their special operations people to grow beards. The idea was supposedly to allow them to blend in. Nice try. Now it’s just a perk. Now the rest of the troops want them. Let the brass sort this one out.