There is, in fact, an iTunes subscription called iTunes Match. That's the one that lets you upload your own tracks to the cloud, scans them to match against the catalog, and then gives you DRM-free high quality downloads of it.

On the other hand, with Apple Music, there's a different subscription that also lets you upload your music to the cloud and scans them for matches, but what you get back out of it is DRMed downloads. Which don't work outside of the Apple ecosystem, and stop working even on it if you cancel your Apple Music subscription.

On top of that, the way matching works is different for the two services. iTunes Match actually analyzes the raw data of the track to do the exact match, while Apple Music seems to prefer metadata. Which means that, with the latter, you'll often get a different variant of the song, in cases where multiple versions exist.

I'm aware of iTunes Match vs Apple Music Sync Library. Incorporating legacy services into new ones is always a product management tap dance of tradeoffs.

My point is that bring your own library of pre-ripped music into a subscription service for syncing is rare these days unless you're over a certain age or are an audiophile (at which point there are better alternatives like Roon). If you're going to do rare things, then there will always be nuances to understanding the tradeoffs Apple took to support you.

It's not rare for anyone who already has a collection of music, e.g. because they are coming from a different service. It doesn't have to be ripped - it may well have been purchased via numerous existing services that sell downloadable files, from Amazon to Apple itself.