As any mortified parent who didn't vacuum yesterday can tell you, a drooling baby picks up dust-bunnies like nobodys business. Having had one to many occasions to contemplate this phenomenon, I've concluded that there's an evolutionary adaptation at work here.
First of all, there's camouflage. On a forest floor, that untended baby would be picking up leaves and small twigs, disguising it from predators whilst mommy and daddy are hunting, or gathering or making more siblings or what have you. Pretty hand really.
Next, when mommy and daddy catch their breath and notice the baby is beyond dirty, they are inspired to clean it. This is good for the baby. I'm sure there were parents who weren't moved to clean baby under those circumstances. Those babies probably didn't grow up to make more babies. You see how this evolution thing is working.
Finally, the mortification of the aforementioned parents at finding baby plastered with dirt inspires a few gray hairs, and shaves a few days off the lifespan, thus shortening the amount of time mom and dad will be around to hog up all the good food. This last element is a process that escalates as a child grows older, culminating in some cases in outright parricide. For examples, see the histories of pretty much any royal family, or spend a few minutes trying to talk to a teenager. Be careful, I'm told they bite.
(Neither the author of this blog nor any of his subsidiary contributors bears any responsibility for injuries sustained while trying to discuss the intricacies of baby drool and food supplies with teenagers.)