Okw does get a panther out in about 16 minutes if they hold their fuel and a large part of the map.
How are they gonna do that without hurtung themselves ? While volks and püppchen will keep shermans/t34s at bay , you will bleed and you will lose mapcontrol in the meantime. and 4 minutes ( as 12 minutes is easily the time medium tanks come out ) is a long time in coh2 midgame.
OKW has the best infantry in the game, you really don't need any kind of vehicle support to deal with other faction infantry (atleast if you pick a doctrine with an infantry call-in, the two current commanders that don't have an overpowered call-in are quite a bit weaker off). Allies on the other hand are very much forced to get anti-inf vehicles against OKW because they just can't compete otherwise, making Panther as first vehicle quite a good choice as it'll still appear before the 10cp stuff that could potentially overrun your volk shrek AT.
And then if they get a panther its not like they got a tank that performs very well as AI.
OKW Panther MGs have a combined DPS higher than an LMG gren squad. It might not kill anything with the main gun but the MGs bleed squads quite heavily.
You forgot that OKW buildings are much more vulnerable.
Nobody is forcing you to place them outside your base, a base sector that happens to have the best defenses of all 4 factions.
FlakHQ has 1920 hp while making all infantry and light vehicle use near it impossible for the opponent. Soviet T3/T4 and Ostheer T3/T4 have 2000hp
T1/T2 trucks have 1600hp. Soviet T1/T2 and Ostheer T1/T2 have 1500hp
The only reason they're more vulnerable is because you keep placing them so badly that opponents can fire at them.