With amazing regularity we see questions from those who are new to the world of model steam and engineering, and having seen the more established designs, wonder why some other construction or design method has not been used. Often they ask "Why don't you do this...?" or "Can an engine be made with <insert mechanical device>?"
The quick answer is usually:
Because it doesn't work.
Because it doesn't work as well as another design.
Because it would be very difficult and/or eye-wateringly expensive for a model engineer to make.
In the two and a half centuries or so that we have had steam engines there's a good chance that someone else already had the same idea (or something very similar) and had a go at building it...
I would encourage everyone with an interest in model steam and engineering to visit the Museum Of Retro Technology where they will see examples of all manner of strange and wonderful engines.