I agree, I would like to swap out an iron as I can do in real golf. For me in real golf I don't carry a 3W, prefer a 2I instead. But the fact is you can.
Jimbog1964: Moreover, I doubt TM would allow it preferring
And I think this is the reason - whether it is contractual from the club manufacturers or as Jim says, subliminally trying for us to buy that next upgrade.(and contractual because why would a company want you to swap their club to use a competitors - and often players may "replicate" their real clubs for their virtual clubs).
So regardless of the real reason, I do not see this changing any time soon.