The reason you can manually change aircraft and not AFVs in WitE is that aircraft do not have fixed TOEs like ground units. The TOEs are designed to keep upgrading AFVs to the latest models so manual change would provide little benefit and AFV production is so much lower than aircraft production that surpluses are less of an issue. Actually to implement manual change for AFVs would require a system even more restrictive than the current one to prevent the kind of abuses seen in SECOND FRONT and WAR IN RUSSIA.
I will agree that manual production could be abused but that is something that should be taken into consideration in how it is implemented, not to mention it is fairly easy to add a game option like "Manual Production ON/OFF" much like Fog of War etc.
So how would adding an option like manual production be easy? And just how would it work? The system already handles the TOEs, which are out of the players hand. With manual production, I, for one, would build nothing but T-34s, Il-2s, whatever the best artillery is. Which would make a mockery of TOEs. And if one has to follow TOEs, then there isn't any point in manual production as the program already takes care of it.
"I always take life with a grain of salt. Plus, a slice of lemon. And a shot of tequila."