Why didn't the Dread Masters make their own Sithspawn like the Monolith?

[Slowpokeking]

All of their monsters are modified creatures with their alchemy or technology.

Kell Dragon, Subteroth, Underwalker, Drouk, Rancor and Trandoshan(Kephess).

 

TFB was some giant monster they summoned from far far away.