Why can't we grow beards?

Even though I served in the Air Force, I've been TDY to quite a few Army bases. The one and ONLY base I've ever been to that allowed beards (actually required them) was, (Classified). They also wore long hair but, it was due to the specific nature of their mission. Unfortunately, I can't tell you the name of the base, where it is or what their mission is but, lets just say...........they needed to be able to "blend in".

“Groooming staaaandards.”
-Sgt. Maj. John Sixta

When I went into the field or downrange I even shaved my mustache off. cleanliness was the reason. A number one haircut was also my choice. If you want sweaty, oily facial hair, go right ahead.

Apart from violating grooming standards and gas mask seal reasons:
No profile that allows it
No mission that requires it
No religion that says it's part of it

That's a very small but specific reason that we kick the shit out of all others....uniformity adds to brotherhood which leads to a fellow brother doing everything he/she can to make sure you and they fight and won and come home...want a beard?? Get out...join a foreign service or get in line and be on the winning team. PERIOD

Gas Mask fit

Your protective mask will not seal correctly

I just got done reading an article that non combatant soldiers ( boots, Kiwis ) were upset that combat vet soldiers were wearing combat patches and they didn’t have any “special “ patch to wear. So, we know not everyone can grow a beard. So, in an effort not to give Boots a reason to cry, no beards allowed.

Here's some interesting info on your question. Like every military tradition, it usually grows from a practical purpose. In this case, it helped Soldiers in combat and prevented the spread of lice. I found this at http://calvusguy.blogspot.com/2012/11/a-close-shave.html if you want to read more.

Of course, Romans didn't just have their nails to worry about, throughout much of classical history, the clean-shaven look was the in thing - until Hadrian came along with a bad case of acne scars and grew the first all-over Imperial beard. There was a practical point to shaving in ancient times too - Roman soldiers shaved so enemy combatants didn't have any whiskers to grab during hand-to-hand combat. And, as a whole, Roman society eschewed all body hair below the scalp (and I mean all hair), initially to combat body lice, but no doubt, as now, because it was fashionable.

Well I know why the Navy did away with allowing beards, Because you can't get a good seal on the OBA's (Firefighting equipment, also it put out pure Oxygen, so beards would catch fire with a small spark after wearing OBA's). Same goes for gas masks. So, it's not grooming thing as much as a keeping you from dying thing.