The WiFi silicon vendors seem to base their SDK/reference software on OpenWRT (albeit often badly-outdated versions), so almost everyone selling a WiFi box ends up using some variant or fork of OpenWRT. It's been a long time since the days of Linksys trying to cut DRAM costs by using something other than Linux. There are still exceptions like MikroTik where the OS and configuration tools are a main selling point (still Linux-based, but not OpenWRT).

Where does Ubiquiti use OpenWRT? I saw that claim elsewhere in this thread but I'm pretty sure their original devices were based on Vyatta, and their newer stuff is a custom Linux OS of their own that is loosely Debian-flavored. Poking around in the terminal all the Ubiquiti devices I have used are very clearly unrelated to OpenWRT.

Ubiquiti started in outdoor wireless, point to point and point to multipoint. All their outdoor radios, cameras, and unifi APs are all running a version of openwrt.

EdgeRouters started on vayatta and then that went away (AT&T acquired Vayatta) and then that forked into VyOS.

Some of the Ubiquiti EdgeSwitches are running some other OS, but I can't remember what it was.

Source - Spent 14 years helping alpha / beta test, debug code and wireless problems, etc.