Yes, installing a service isn’t particularly complicated. It just takes writing a handful of registry keys. You can have a look-see with Regedit.exe, navigate to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\services.
Sc.exe can write these keys too, using the supplied command line arguments. Nevertheless, this is not the right way to do it. The point of InstallUtil.exe is that it can activate custom installation code. Code that the service author wrote. Which is not that uncommon, services tend to stuff config info in their registration keys for their own use. You’ll see plenty of evidence for that when you have a look with Regedit.