Because its triggered through emotions caulifa is just an exception. Goku and Vegeta didn't know how to trigger it off the bat, didn't gohan get ssj by training and two through anger? Didn't trunks also?
gohan went through hell in the time chamber and had a multitude of fights and deaths before he could tap into ssj power.
trunks had to see his lad shred of hope die (gohan) before he got triggered to transform
kid trunks and goten got ssj because it was passed down by genetics