It's not that it's not doable, it's that their design and the components used tend to be problematic to obtain.
The quartz tube, for instance... not that easy to find laying around. UV also degrades most plastics, so the plastics used have to be specially made to withstand exposure to the UV....
It's one of those things that getting the parts is such a pain, that they aren't really that practical to DIY.
Ozone should be doable, but again, you are dealing with Ozone's tendency to break down anything exposed to it unless it is special material, and the high voltage generator is something only somebody well versed in electronics and comfortable working with the thousands of volts involved should do. The other thing about ozone is that the ozonizers on the market are calibrated for the amount of ozone produced. Most people don't have the equipment lying around to measure an ozonizer's output, and too much ozone can wipe a tank out.