That's not true at all. Synology will sell you 24 bay rack mounted devices and 12 bay towers, as well as expansion modules for both with more bays you can daisy chain to them.

Granted, I believe those are technically marketed as enterprise solutions, but you can buy a 12 bay unit off of Amazon for like two grand diskless, so... I mean, it's a thing.

Not saying you should, and it's definitely less cost effective (and less powerful, depending on what you have laying around) than reusing old hardware, but it does exist.