if you are putting $12/mo into a big pot via Spotify there's way less money available to pay artists than if you buy a few albums outright a month, it takes a lot of Spotify streams to earn an artist a dollar. If you stream an album a few thousand times on Spotify maybe it works out in the artists' favor, I suppose.
and crucially, it's not taking 70% of your $12/month and parceling it out to the artists you listen to, it's taking everyone's money and putting it in a big bucket and parceling out it proportionally by stream numbers, so you can't direct $8.40 to an artist by only streaming them.
If you want to direct a few dollars to an artist you really do need to buy their music (or mech), or somehow give them a couple thousand streams.