Linux development is paid for either directly or in-kind by companies including Red Hat, IBM, Canonical, Oracle, and others. It's free to use and mostly open source but if it existed only on passion it would be something far less than it actually is.
People need to eat and have a roof over their heads.
Those companies pay the improvements they want for their usage case, which is usually far removed from what normal users want. I don't really need support for thousands of CPUs and terabytes of RAM.
Do you remember what Linux was like before the big corporations started contributing/supporting it? Just getting X11 working with your video card and monitor could take hours or days. Setting up a single server could easily be a "project" taking weeks. And god forbid you ever had to update it.
That in particular was thanks to the X.Org foundation. And while it made things easier, it didn't take "days" setting up a graphics, it took hours at most. And setting up a server didn't take weeks, it was an 1-2 day task at worst.
People need to eat and have a roof over their heads.