Because that would require manual work (reporting back the results), maybe if we figure out some way to dispatch the request to run tests with a PR against an environment.
If we had a repository in which developers could register their environment to receive requests to run tests on a given PR, and then store the result somewhere to be checked by some git hook. When a PR is created a hook dispatches test requests, and then other hook checks the results of those tests (instead of reporting back, we consult the result of functional software tests).