I would love to see non-wheeled luggage for sale in, well, wherever luggage is sold. Wheels are unnecessary on weeknd-and-smaller bags. And yet, people are lazy; therefore, I'm sure luggage w/o wheels will become even more scarce.
There is no good reason why all purchased luggage should contain wheels. Certainly, the size of the item is the most important parameter. I would suggest that most items of overnight size, and smaller, would be perfectly acceptable without wheels.
Those wheels can really be an obstacle! Aside from the fact that they might make the suitcase not fit in the overhead compartment and therefore not qualify for carry on, the presence of those wheels might enourage you to pack much more than you should. The saying "carry twice as much money as luggage" is something to consider.
Luggage without wheels is much lighter than luggage with wheels and the wheels take up space inside the bag. If you're strong, you should not need luggage with wheels.
Yes, you do not need to buy luggage always with wheels. There may be some places where luggage with wheels looks awkward. Also it also costs less. You can actually add wheels to your luggage later on if required.