What about having their own sniper would make them OP?
The other three factions have multiple sniper counters (222, UC/AEC, Scout Car/T-70-etc), along with having their own snipers for counter-sniping duties, I don't really know why the addition of this unit to either of these faction's rosters would cause them to be overly strong. USF and OKW lack the unit altogether with no discernable tradeoff.
The Kubel is very bad at chasing units, it is effectively a casemate, has poor survivability, and (Currently, though this is improved in the next patch) abysmal on-the-move performance. You can't really dive past other units with a Kubelwagen to chase a sniper down. Its detection ability is fantastic, however.
Because OKW and USF have Pathfinders and Jaegers. There's no real reason to have a sniper if you already have long range ambush infantry as opposed to short range ambush infantry like Storms and Commandos. Besides, if you made the sniper doctrinal, then the rest of the doctrine would have to be kind of shitty because a sniper by himself is pretty fucking strong. I mean, just look at the Wehr vs. Brit meta. It's literally just the Sniper carrying the entire faction on his shoulders.
Also: The other two factions don't have the same level of elite infantry that OKW and USF have. Falls, Obers, Paras, and Rangers are barely comparable to Guards, Commandos, and Shocks.