Because in CliMulti we might check in a loop for every process constantly if they are finished. It won't help all that much but better to not check every time the file system when it is not needed.
There be many other things we could improve there. Will add some comments but probably best not to do them (or we'll see if it's needed later)
Technically, we could also set
$this->started=true and avoid filesystem checks when
startProcess() is called but shouldn't be needed at least in current code usage. It wouldn't really avoid any file system checks so far and would make it only harder to find some bugs maybe as the filesystem be avoided in the tests potentially.