Tucky, we could go back and forth on this all night...the bottom line is no one wants the added hassle purely for a free room.
Even if I changed a couple of times in a day, it's still the same short, wild-haired platinum blonde taking another gent upstairs, whether I'm wearing jeans or a black dress...even waiting at the bar only works so many times in the same time period.
It's not being lazy, it's simply a huge inconvenience, and not only for the lady, for the gent as well. Often times guys get lost looking for an elevator in a place they've never been to (even with explicit directions), and it becomes very obvious what's going on very quickly.
The gentleman I see are also very much against anything that would compromise discretion. We aren't talking about necessities here such as electricity & transportation. This is a luxury item and I, as would the large majority of other ladies, rather concentrate on our date details than how to just get them up an elevator. If we're both frustrated over the ride up, the ride behind closed doors will suck!
You're right, it's not rocket science, and the most efficient solution is to avoid the hotels that require key cards for elevators ; )