Does anyone knows why Wi-Fi routers vendors keep writing (and maintaining for a short period) own firmware which in most cases has lower quality than OpenWRT when they can use OpenWRT and save money?
Because router vendors need some kind of "secret sauce" in order to keep selling routers, otherwise the whole thing becomes a commodity you can buy from anywhere. This is also why OpenWRT is unable to utilize all the features on a lot of routers: The closed source firmware and software pieces needed to do so have yet to be deciphered.
Vendors can ship OpenWRT + closed source drivers. Anyway they use Linux kernel (yet to see a closed source BSD router; PfSense is occasionally used but its open source) relying on GPL exceptions allowing to use closed source drivers (and firmware blobs).
Actually, there is a large number of manufacturers using OpenWrt as a baseline for their firmware. Unfortunately, these are often outdated version of a forked OpenWrt with heavy modifications to include firmware blob for hardware acceleration and such.