They're rare enough that lowering the spawn rate would do nothing but enrage players. Also, did the 1.3.0.2 hotfix fix the Truffle Worms not spawning, or is that bug still persistent? I know it was a bug with the original 1.3.0.1, but I don't know if it was fixed in 1.3.0.2.RNG strikes again. Keep trying.
Either that or they lowered the spawn rate, but I don't really know.