For good reason. The size limit is the size limit, I don't see what adding two wheels would change about that. For anything that fits in the carry-on size limit, the wheels aren't about the weight, they're for convenience.
If people can't lift their luggage to the overhead bin, they're either weak or you need to find an airline with more reasonable overhead bin limits.
The wheels are definitely about the weight. They promote packing heavier and more, since the owner doesn't have to physically carry it through the airport. I bet you most of those wheeled bags are 40+ pounds.
I have a piece of luggage that I love. It is, to within an inch in each dimension, as big as any airline I fly will let on as carry-on (I've seen people carry on larger, but mine is within the stated limits). I can carry it fully packed to the point of nearly bursting, and have done so when one of the wheels was broken.
But it has wheels, so if they're functioning, why shouldn't I use them?