I can’t disagree with what your argument here. You could make a case for just about every single unit on the board that it shouldn’t be allowed until a certain date. However, to make the game playable without having to refer to a reference sheet or rule book with every single move you make, it has been simplified somewhat for ease of play.
A number of units have been singled out and for the most part separated from the regular units and put on the tech chart. Tanks were also singled out and had a date affixed for the obvious reason that they are so powerful with their ability to blitz. Players are welcome to create house rules to make it more complicated or less complicated to match their liking.