I suspect that a lot of it's just historical happenstance. Podcasting as we think of it today started ~2003 and was very intertwined with the development of RSS. The click-wheel iPod (which was really the breakout model) wasn't until the next year. And the current era of podcasting didn't really take off until it became easy to sync with smartphones.
So this was a case where Apple sort of came along for the ride and didn't see podcasting as being a big deal--which they were sort of right about; arguably even today it's somewhat mainstream but I'd bet the majority of people in the US have never listened to a podcast and certainly don't listen regularly.
Yeah another thing I'm surprised at, being a relatively new podcaster, is how often I approach people/businesses to interview them and they tell me they either don't listen to podcasts or don't even know what they are.
On a recent trip back home, I had some downtime and my device was broken, so I threw a bunch of podcasts on the Apple TV.
A few weeks later my dad calls to tell me he doesn't know how any of this works, but now he gets amazing content while driving, instead of just listening to the mostly ads that came through his antenna.
So.. just lack of awareness? Just fiddly enough to keep people away?
I have a sneaking suspicion that if tomorrow Apple or Google just subscribed everyone to 99% Invisible and waited, FM radio would be dead in a year.
Probably all of the above. Plus, for a lot of people, radio is mostly background noise that, in the case of news and talk radio, is always current. And there's always NPR to mostly avoid advertising.
Many people don't want to have to deliberately choose content a lot of the time. I've heard people argue that they don't want to always have to use Netflix because they can't just flip on a channel and/or channel surf.
So this was a case where Apple sort of came along for the ride and didn't see podcasting as being a big deal--which they were sort of right about; arguably even today it's somewhat mainstream but I'd bet the majority of people in the US have never listened to a podcast and certainly don't listen regularly.