I am astonished that someone who has been a Marriott Platinum for two years has never gotten upgraded.

I usually stay in Fairfield Inns, and there's not much they can do for you there, but even there I usually get a "suite" or something.

It may have something to do with, for want of a better term, "agressiveness". If you never do anything proactively, your chances of getting upgraded are less than if you call ahead, or mention it to the desk clerk (with a printout of the Marriott T&C's in your hand), etc.

If you just passively show up and hope for the best, you may never get it.