Not everyone was as pure hearted as Virvor. Therefore, not everyone would follow the vow they made. In fact, Dyon was certain that a good portion of them would directly ignore this promise and build their own forces.
It may seem stupid for the Celestial Deer Sect to not bind these individuals with contracts, however, think about it. Would you want to build your sect or clan off the back of individuals who were forced into joining against their will?
This wasn't all either, the elders of the Celestial Deer Sect, physically speaking, couldn't prepare contracts. Remember, Elder Daiyu had his grandson, Chenglei, sign a soul contract with Jade instead of him because his soul was severely injured.
If a person who was alive and injured couldn't withstand a soul contract, then how could a mere spirit withstand it?
And, considering they didn't have live bodies, they obviously couldn't use blood or blood essence contracts.