Actually the pvc has a fairly thick wall. Combine that with the tubular construction, and a relatively short linear distance, and you get a very stable piece of equipment. I used a fairly thick piece of pipe, and mine are 22" long. I am about 205, my buddy is 230, and we both stood, and bounced on one of mine. No problems at all. You have to remember that they are not subjected to the dynamic movements that a set of competition bars would be.
The design of pipe, a cylinder, is basically a continuously convex surface, and is actually better at resisting external pressure than internal.
Mine are glued everywhere but the top of the T's that make the base, to make them more portable, and they never move at all.
There is always the option of using black iron pipe. Slightly more expensive, and very unforgiving on the body. Not to mention heavy!