I'm pretty sure it comes down to radio vs interactive music licensing. A general radio license doesn't allow users to pick the track they want to hear. They only get a shuffled playback and face other limitations like "can't play an album all the way through". Interactive licensing allows users to pick exactly what they want, including playing full albums, but it's much more expensive per track.

It appears that Spotify's engines use a mix of these licenses to reduce costs. Since AI isn't explicitly user-made selections, it's quite possible that the AI playlist generator is limited to a radio license model for playback, simply to save money (considering the additional cost of providing AI).