Main reason is for you to only use cloud storage and services. Google doesn't make money if you own your music and play it at will from an sd card. Google makes money if you have to shell out $10 a month to play the same music again. Replace <music> with anything else -- works the same.

Big tech wants you to not own anything, because in that case you'll have to subscribe to everything. If you can't subscribe or you don't have enough money to subscribe they'll make it "free with ads". But one thing is a no-no -- you buying and owning any content.