No, they aren’t. Of course anyone can put up a podcast, but the successful ones tend to cluster into podcast networks of like shows or join the big podcast groups like Nerdist, Wondery, or Earwolf. Or be a part of an established media company like NPR or Slate.
It’s easy to start a podcast but very hard to survive and expand alone.
Within a margin of error I'd imagine virtually all tech centric web content has at some point been sponsored by Audible (Amazon), Squarespace (private), Dollar Shave Club (Unilever), or rarely one of a few VPNs like Tunnelbear or Private Internet Access.
Organisms do find it easier to survive in groups, but the main point here is that even these groups like Nerdist, NPR, etc. would still have publicly available working podcast feeds if somehow iTunes stopped working tomorrow.