Which bathtubs do not require installation?
I've never heard of such a thing, and I can think of a number of reasons why:


Bathtubs are heavy. Even worse, water is heavy. A standard bathtub holds about 40 gallons of water, which is over 300 pounds. Add the weight of the bathtub itself, and you may cause damage to whatever it's sitting on.
Showers these days are designed to not use a lot of water (about 2 gallons a minute). Take off the shower head and you'll get more, but it's still going to take a while to fill that bathtub. And, when you're done filling, the water is likely to have cooled off.
How big a water heater does your apartment have? Big enough for a 10 minute shower isn't big enough for a bathtub.
Where would you put the bathtub, both during and after use? Unless your shower is enormous, or you have a really big bathroom, it's tough to imagine how it would fit.


Add all these together, and the many ways a customer could damage his home and then blame the manufacturer, and it would be tough to make money selling standardized portable bathtubs.