We see the four spirits we’ve seen so far in the saga, is the ultimate reason. Leia has not yet fully transformed into the Force evidently, and as Jar Jar said above ROTS implies the ‘disappear and become a ghost’ ability was recently discovered in history. (So weird.) in the theater, the voices made me question things, since it would seem to go against ROTS. (Although the novelization mentions Yoda being chill about Qui-Gon’s voice because he assumed it was merely the Force speaking to him using his voice until he emphasized “no it’s really me.”)

Also, having four we’ve seen before feels better than 37 all over the place.