Or (IMHO) the most probable presumption is that rats are not at the peak of their evolution and this particular trait just hasn't occurred yet or would not be a naturally occurring mutation.
Your presumption that "if they haven't evolved to do this yet then there must be some disadvantage" is inherently false.
To extrapolate further, using your presumption, "if rats were humans they'd have an evolutionary advantage, why aren't rats humans". I would imagine one could list a million "simple mutations", each giving an advantage, that would lead to a rat having all of the genetic advantages of humans. Why haven't each of these in the chain occurred?
Evolution is not necessarily really even about selective advantage or survival of the fittest. It's about who reproduces the most (which is not necessarily the fittest or most advantaged). In fact, evolution only seems to really accelerate in extreme conditions.
Anyways, I'm going on too far, but I reject your presumption that a genetic advantage must necessarily have a disadvantage if it isn't naturally occurring.