The reason is growth. A sysadmin will make sure the lights stay on and the place doesn’t burn down, but they aren’t creating a new revenue stream. And when all these companies need to grow forever, they need to be creating new things, which developers do and sysadmins don’t.

And that is why platform/devops is safer than the sysadmin. Not only do they help developers create new things, but make it complicated enough that you can't disband their team ... {tongue in cheek, sort of...}