I’m guessing it is because devs might need to have one version of the engine installed per project.
Game devs are probably more interested in having everyone on the release they plan to ship than in continuously updating to the newest version of each dependency.
Apt doesn't really support multiple versions of the same software installed side by side. Other package managers like Conan and Nix do.
If your software needs to support multiple versions installed in the same user space it doesn't make sense to support package managers not designed for that use case.
Game devs are probably more interested in having everyone on the release they plan to ship than in continuously updating to the newest version of each dependency.