Why should they? Not saying they can't, but I don't get this presumption that every faction needs at least 1 of everything (see: constant Pershing begging). Why shouldn't factions have intentional unit gaps? Diversifying the factions and their playstyles makes the games more interesting.
If they really, absolutely need it to be viable then sure, but just "the other guy has it" is pretty much the worst reason you could give.
That's all well and good, but it kind of feels that USF is missing more "stuff" than the other factions.
No stock "elite" infantry (I know Penals are meh but they fit the bill)
*Kinda sorta countered by Rifles being the best stock infantry, but Rifles can't stand up to Axis elites outside of doctrinal LMGs.
No heavy tank
* Obvious. Jacksons are strong, but whether that is enough is up for debate.
Lack of mines
*Only faction with mines locked behind a tech choice and no anti-infantry mine at all. The doctrinal light AT mines are bad, and I think bugged as well. Could counter by saying USF is only faction with non-doctrinal tank traps. I'd trade any day.
No Rocket Artillery
* No real mortar either, just the overpriced Pack Howie and the terrible doctrinal Halftrack. Priest is supposed to fill this gap I guess, but is doctrinal and in my opinion worse than the Katyusha and Stuka (but better than the Panzerwerfer).