Two reasons:

1. It’s a bit orthogonal. APs that support mesh aren’t actually taking their excellent 802.11r/k/v network and making the APs themselves use it by mesh magic. They’re doing something else behind the scenes to arrange for wireless uplink. You can set up OpenWRT to do WDS (I think) and get it to have a wireless uplink. It will be mildly annoying to set up, it will be more manual than it deserves to be, and it won’t help get the actual multi-AP SSID to work any better.

2. I said that I, personally, didn’t care so much. I’ve set up several wireless networks, and I’ve almost never wanted a wireless uplink. I like wires!

OpenWRT probably should have better out of the box support for “mesh,” but IMO that’s a different feature request. I do find it rather annoying that AP makers tend to conflate “mesh” meaning wireless uplink and “mesh” meaning you can use more than one AP, it works well to do so, and it’s not unnecessarily painful to do so.