From someone that lives in a country where silencers are not regulated, silencers are still expensive. Like most things, you get what you pay for. The cheap one from the machine shop is usually not as quiet, light or short as the expensive import.
And yes, any competent machinest can turn these out, but it takes a while and what is your time worth by the hour? You can speed things up by using CNC, but then you also have to pay for the machine running costs and maintenance.
Lastly, it is not just a matter of throwing baffles in a tube and hope it works. Some designs work better than others. And the better designs are usually patented. Normally not good bussines sense ripping off a patent and getting caught.
Trust me, many of them here are.
You can tell if a design was submitted for a university or external specialty firm consultation.