Generally, motherboard manufacturers source their components from other companies. They do not manufacture the entire board themselves. This includes CPUs, Wifi cards, USB controllers, bluetooth, audio, display controllers, etc. Each and every one of them create new products, maintain their own firmware for all those new products, and push updates to the motherboard manufacturers when there are updates.

Coreboot/libreboot do not update those components themselves. They also must be provided that source code.

Just for coreboot alone, the last release had more than 120 contributors push over 900 commits. One person is not able to maintain that piece of software, as it is an enormous task.