Unfortunately common practices in many hobbies are rather dangerous. I cringe when I see dangerous practices in hobbies I'm familiar with, and I freely admit that I've done the wrong thing before out of ignorance.

Another aspect of the problem is that while many people are diligent about using pressure-rated pipe, they accidentally end up using non-pressure-rated fittings. I myself have fallen into this trap before in some projects of mine, and I didn't recognize it until I started learning more about potato guns. When making a project like this I typically go for all metal construction when possible because the components are stronger and the pressure rating is usually easy to find. For plastic fittings you need to familiarize yourself with the various markings that indicate the fitting is pressure rated. And even then that doesn't tell you what the rating is. In my experience, the pressure rating of a fitting is always lower than that of the pipe the fitting attaches to.

When I was a kid we built tennis ball cannons out of soup cans duct taped together. You take off the top and bottom of 4 cans and smooth out the edges. In a fifth can you punch several holes in the top and one matchstick sized hole on the side. Duct tape them together with the hole can on the bottom. Then you push a tennis ball down the barrel, squeeze a few drops of lighter fluid in the hole, swirl it around a bit and hold a lighter up to the hole. Done right it will chuck a tennis ball a couple hundred yards. I guess it might explode and shred everybody nearby with sharp metal can fragments, but the whole thing is pretty low energy so probably not.

FWIW, the fittings I used in my teenage-years rockets were usually gypsum or plastic + lots of duct tape (for the rockets made from vitamin fizzle tablets containers); their pressure rating was "looks like it should survive the launch, but we'd better move back a couple meters and duck behind cover, just to be sure").