It's significantly more expensive if you listen to a wide range of music. Buying albums, or even just individual songs, can very quickly surpass the $10/month you pay for Spotify. Maybe that's worth it to you, but I can't blame most people for just paying for a subscription.

The issue is control. Albums are regularly disappearing from the Spotify catalogue, sometimes reappearing later. Sometimes specific albums in a band's history are missing because that particular label doesn't have an agreement with Spotify. And you never know if it's a crappy modern brickwalled remaster. Not to mention the woefully bad curation, mixed up artists, incorrect genres/years, you name it.

With music you buy, you are in control.

It's about quality over quantity.

Same with video streaming. With Netflix you aren't guaranteed the show won't suddenly just disappear before you finish it -- happened multiple times for me. I've been partly going back to Blu-ray for better availability and picture quality. Netflix is still very good for exploratory watching though, but Blu-ray is very very good for stuff like classic HBO series and BBC nature documentaries.

I've seen some people suggest buying one album per month and then pirating everything else (or streaming everything else). Best of both worlds, sort of.

If you listen to a wide variety of music, likely some of it is more important for you, something you don't want to lose.

This is the stuff to buy outright.

Exploratory listening without such a commitment (youtube, spotify, bandcamp) is of course very important, and is a blessing.

I bought my first music on Bandcamp a couple of days ago (they had a day when 100% of sales went to the artists). That seems like one of the best ways to support artists while still having a cloud-hosted library.