Greater Daemons are pretty common though. There are more Bloodthirsters than Adeptus Custodes. Its only the Exalted Greater Daemons that are rare. Angron has 12 Bloodthirsters as a body guard, multiple Greater Daemons and a Daemon Primarch is a fluffy army.
Daemon Princes are way rarer than Greater Daemons but used to be spammed all the time. The 4th ed 'Heavy Support slot daemon prince' rule made no sense and confused a lot of people.
A Greater Daemon is just a regular Daemon general, the equivilant of a Captain not a Chapter Master. Because there's no rank between Greater Daemon and Chaos God there's a big continuum of rank within the Greater Daemons. Ku'Gath was Nurgle's commander in a large warzone during the Ultramar campaign but his lieutenants were also all Great Unclean Ones.
If Soul Grinders were worth spamming we could see Daemon target saturation. They could also add in more Heavy Support Daemon engines for the faction.