I think companies like Spotify end up spending 60-70% of that $12 on paying license fees, which include per-play amounts that eventually end up with the artists. And the structure of those deals is such that the groups with the biggest licensing deals get most of your $12, even if you didn't listen to music from that particular artist.
I agree that it's not what you (or I) want, but it does make sense from the perspective of the largest parties involved in these negotiations.
I think your point is pretty valid though, if I listen to 6 hours a day I am contributing way more towards band payouts than someone who listens to a few songs a day, even though I am putting in the same amount. The direct connection could actually encourage people to subscribe, have a page showing your contributions to each band you listen to.
I pay $12 a month and only listen to one record three times through on that month. You'd expect the artist(s) of the record to get my $12 minus a service fee, but what I'm sure happens is the artist(s) only get the price per play x 3 and Spotify pocket the rest.
Subscription models are a huge boon for this reason, but the inequitable split might change artists' minds about these sub services.
I tend to listen to a small subset of music over and over, the artists should be getting close to that $12/month cost in my opinion.