Funny you should mention PostgreSQL, since it includes a monitoring process - the postmaster - that restarts the workers if they die ;) (and they do, occasionally).
That's because the child processes can load and run third party code, which obviously can crash them. Just like apache modules can crash httpd worker processes. So why run an extra layer of monitoring on top of the existing one for apps like those?
All the software in the poll is general purpose monitoring/restarting software. It isn't software designed to be multi-process and monitor its own child processes.
