No, it's because every call-in vehicle has a unique power level that doesn't always fit into the stock tech tree. Unique solutions are required to balance their arrival. The M8 Greyhound is a good example for this, as it's similar in power to a T-70 while USF has no T3 tech, which means it would arrive too early in CAP/LT tech and too late in Major tech. So the only solution (while keeping the unit strong) is to put it behind 4 CPs, even if that means it generally comes too late in teamgames, so at least it's strong in some modes without arriving too soon.
I don't mean you are wrong, but according to explanations from the period of time, when all of tied to tech changes were applied (except the british Lend-lease commander M10 and Ostheer P4J), the reason was to avoid strategies, when players ignore tech and rush for call-ins. Now, when balance team find a smart solution with call-ins + tech requirements + CP, it will be more logical to bring the rest of doctrinal vehicles to same standart.
Additionaly, all mentioned tied to tech vehicles were OP mainly because they allowed to play without investments in tech, so timing wasn't a real issue here (or not the main reason to tied them to tech).
As a result:
Relic Entertainment
