Most people don't want the hassle of running subscriptions. It's not as simple as putting a Patreon or PayPal link online - as soon as people become paying customers they start demanding things. It takes a lot of the joy out of being creative and turns it in to a job.

Having one sponsor like Spotify means you get paid but also retain a lot of freedom and distance from the audience. If I was a podcaster I'd much rather take that route, even if I lost some listeners.