This could be said for literally every faction in the game dude. The reason you never see many types of units from any faction is a little thing called "The meta". Unless you give OKW more viable options you are not going to see more.
People want OKW to have things like indirect fire, more selection in vehicles like actual medium tanks and what not so they have more options to play around with. Soviets are one of the most fun factions atm due to how many units and indirect units there are to play around with. Ostheer would be the same way if it wasn't trapped behind horribly expensive techining.
I think OKW is a shitload of fun personally, but that's only because I do odd and crazy things like use Sturmkittens.
About every faction?
SU: Cons, Shocks, Guards, Penals, Sniper, Maxim, ZiS, T70, T34, SU85, Katy, IS2, 120mm.
OST: Grens, PzGrens, MG, Mortar, Pak40, PzIV, Tiger, 222, 221.
USF: Rifles, Lt+M20/AA or Cpt+57mm+Stuart, Shermans, Jacksons, M8, Paras.
We see those units all the time. One time it will be Sherman spam, other it will by Jackson+M8.
OKW:
Volks, Obers, Luchs, Panther, King Tiger. Most seen OKW units. All 251, leig, Panzerfusi, Falsch, Jaeger, Puma, Strumtiger, raketen. You dont see this units often.
Yes, sometime, someone will try to go for Falsch+Jadgpanzer but then he will realise than Obers+Panther are just better.
It's just shitty design of the faction.
SU or USF you must decide if you want little support (T1, Lt) or heavy one. This must be combined with your doctrine, like snipers+guards.
OKW can give a garden about that. No matter what will enemy do, Volks+Obers+Panther can stand against everything.