Wait, really? By who? For what reason?
Red Hat, Intel, AMD, SUSE, Samsung, Canonical etc.
Obviously, a lot of paid work is done on low-level plumbing, because Linux is hugely profitable as a server, infrastructure, or embedded operating system for a lot of companies. Graphics infrastructure (KMS, Wayland, X11, etc.) is useful for companies who embed Linux in various appliances (set-op boxes, etc.). Red Hat probably invests in GNOME because for Red Hat Enterprise Linux Workstation and dogfooding.
>For what reason?
If your programmers can make your project their job, you don't have to worry about the amount of time they have to contribute.
People working on most components of the desktop Linux stack are being paid for it. Think kernel -> video drivers -> systemd -> wayland -> GNOME. All of those groups have paid employees. They are evidently understaffed.