They come from employees of various companies, not employees of some company that owns Linux itself. All those various companies have different goals, and are only contributing because doing so is in their economic interest. Having a stable ABI would require a lot more work, and these companies have zero incentive to invest in this effort: it doesn't help their profitability, it just makes things easier for others outside their companies. Arguably, it also makes the kernel code a lot more complicated and bug-prone.
It makes sense for MS, for example, to want a stable ABI to make things easier for 3rd-party devs so they'll target that platform, and for MS to shoulder the effort of maintaining that ABI cumbersomeness. It doesn't really makes sense for Linux. You could argue that this hampers adoption of Linux as an alternative to Windows on desktop machines, but even if that's true, no one involved really has an economic incentive to change this. In the places where Linux is dominant (Android + servers + embedded), a stable ABI isn't really helpful or needed.
Of course Redhat can put what they want into their distro, but there are a lot of choices beyond that.