I have many nerdy friends who have been Linux users for ages. But most of them don't know such a thing as Openwrt exists or have never bothered to give it a try.
It's a very fun piece of software to play with and can be extremely useful for routing traffic.
Wondering why it isn't more popular/widely used.
I've been using it for years and now I basically can't live without it. I consider OpenWrt compatibility in all of my router purchases. Currently using a Netgear R7800 and a Belkin RT3200, both are going strong.
It isn't as widely used because it can be finicky to flash sometimes, and that's if it's even compatible in the first place. Even if it works, you may experience a drop in performance unless OpenWrt supports using the routers hardware acceleration features. If there's no support, OpenWrt basically uses the onboard CPU to do routing and they're usually not all that powerful.