Masticating a bolus of chewy grains is one of the least rejuvenating corporal experiences imaginable, on a par with swathing your body in a wool blanket.
As they grind and masticate their food, they need longer guts to help digest it while meat eaters found having trimmer tums gave them an evolutionary advantage.
Other spiders with more powerfully built chelicerae masticate the entire body of their prey and leave behind only a relatively small amount of indigestible materials.