Why ? Oddly enough, for exactly the same reasons as recycling previously recycled dross : laziness and ignorance. Why bother to learn about exactly what each spice can add to a dish when you can just copy whatever the previous chef used to put in his mix powder, leave out a few of the more expensive or hard-to-source ingredients, then sell the resulting "curry" for more than he did to non-discerning punters who have already nuked their taste bugs with six pints of King Cobra.
Also (to be fair to the odd chef who does care about quality, and as later contributors have pointed out), it is far easier to get an accurate blend by weight when you are measuring in spoonsful than when you are measuring in milligrammes in the context of a commercial kitchen.
** Phil.