I think that it is less about the ad revenue and more about decreasing the total proportion of streams that they have to pay royalties for. Spotify pays people to churn out AI music that they own, and they place that music on their popular playlists (to increase plus from actual listeners) and also have bot armies streaming it endlessly. Since royalties are not a fixed per-play fee, but rather a calculation based on the total amount of subscriber money divided by the total number of plays, massively increasing the total play count with music that does not require royalty payments means they pay out a lower proportion of what they take in each month.