Don't know--perhaps because (this is pure speculation) it was introduced when colours were introduced? Polish and cochin were the most common crosses used (and still used). For polish the knob (aka vault) is required, and its absense is indeed a DQ.
And yes, why have it if it doesn't affect the crest size or feathering is exactly what those who breed against it say. I am not aware of any actual studies or research to verify this. It would be interesting to collect the data from all poultry shows for a year--count silkies with medium to large crests and check to see how many do and don't have the vaulted skull.