The location of the data isn't known, and the Knights should probably learn to respect humans as a fail safe, so there's that.
Plus the mission module works better when the humans are involved.
Also I'll admit I never liked them being called "tamers". I mean, Digimon can speak English/Japanese, and are sentient, so can they really be tamed? I don't think so. More like befriended. Reasoned with, yes, tamed, not really.