It's can be difficult to understand by big companies write so much code, but it becomes obvious once you're inside of one: business software can be arbitrarily complex, because businesses can be arbitrarily complex. The guys in suits earn their paychecks by constantly coming up with new things the business could be doing.
"All" a kernel does (for some very large value of "all") is schedule userspace programs and manage the system's physical resources (memory, disk, devices). You can reach a point where a kernel is done, in the sense that it meets those basic needs with an acceptable level of performance. Kernel developers don't make extra money for every new feature they add - if the system is good enough, then it's good enough.
