As I have been involved with OpenWRT/LEDE and how they make builds to different devices recently, I realized that making unified builds are actually possible. OpenWRT has the same kernel and root image for many different devices that use the same SoC. So once you have OpenWRT flashed on, the content of every OpenWRT update (for every device of the same SOC) is by and large the same. The only check in place is the artificial model check to make sure you don't unintentionally flash the wrong image.

Android can technically do like OpenWRT does, but there is much more stuff involved in the Android world so making a universal build is harder. However, I personally can't think of a hard, technical barrier. New ideas for unifying devices and separating blobs such as device tree or separate vendor partition is getting traction and makers such as Sony are embracing it. Many Sony devices boot the same kernel, and that's why you consistently seeing Sony devices getting support and builds very early on from projects like CM.

BTW, I have been trying to write an answer to you by saying a slightly different version of what dispose54312 said, but then realized it didn't make sense. I came to the conclusion (for myself) this is mostly "how it is done currently for Android." So I hope my unpopular answer is not crazily misguided, and please take my perspective with a grain of salt.