Short answer is $$$$.
Longer answer is: Spotify must pay royalties for each song played. Imagine if there was a completely free (for Spotify) form of content that filled users ears for hours, thus removing the need for Spotify to pay royalties. Ahem podcasts. Now imagine if Spotify started injecting ads into said media form to grow their revenue beyond subscriptions. Again, podcasts. So now, you have a very long-form content that both saves you royalty $ and drives new revenue. QED. Podcasts will continue to be plastered all over your recommendations, be top search results, etc, until the above stops being true.
Wrote about this in detail recently: https://www.towardssoftware.com/spotify.txt