Web apps generally need to be accessible on port 80, and only a single service can claim an IP address and port combination. If web apps were distributed as services, each one would need to run on a unique IP/port combination. In order for outside users to access those apps through the default HTTP or HTTPS ports (80 and 443, respectively), you would still need to be running at least a reverse proxy server that could remap incoming traffic to the correct internal port. On top of that, your web app would have to include an FPM or UWSGI service, increasing the size of the distributed binary.
Frankly, it's much easier just to leave that aspect of things to the server admin, and distribute your web app as a PHAR or Egg or Gem or whatever.