>They require time, and expertise to produce, not much money.
At some point though, time and expertise are money. I'll do an interview-style/conversational podcast as a hobby or as one of the many little side things I do for my day job. But you start talking about multi-day production efforts and that's not something I'm just going to do for free.
Conversational podcasts about some niche topic and Radiolab both have their place. But there are real costs associated with production values.