Why can't US Army soldiers use umbrellas?

AR.670-1 wearing of the uniform.

There is not always a set number of squads in a platoon, or platoons in a company, etc. The link will take you to a website with diagram that provides information on how Army operational units are organized - from the Theater Army, Field Army and Army Group level, down through Corps, Divisions, Brigades, Battalions, Companies, Platoons and ending with Squads & Sections

I've seen people use them in dress uniforms but we have wet weather gear for our ACU's. Check AR 670-1 wear and appearance of the military uniform if it isn't in there then there is no reg.

They usually wear Rain Pancho's, or what ever the military calls them rather than Umbrellas.

Because they wear waterproof capes, the same as the British Army. The same cape can serve as a groundsheet, as well as a one man tent.

I've seen enlisted personnel and officers using umbrellas before, but that wasn't on base. I am not sure of the regulations exactly.

Maybe all the ones you've seen just don't have one?

It isn't issued as part of their uniform nor equipment. It would be funny to see sailors using umbrellas on board ships.

ive seen them and they are always black with no writing on them

Interesting question. Don't know the answer. Could it be that they can block vision?

umm... because they'd look gay?