There is a rover team, supported by an army of scientists, who determine and execute every inch of movement of the rovers. They plan for days over tiny treks of meters, looking at every element of the surrounding surface.
Why would these extremely capable folks overlook something as obvious as water, or potential water, on the surface? Maybe with their greater knowledge than ours, they know what "is" and "isn't". Despite what it "looks" like...
This is their life, it's what they do. Who are you or I to second guess their knowledge and efforts? We are mere amatures. I can only suppose that, had they even suspected some evidence of water, frost, vapor or gas, they would have gone there immediately. Think of the potential...really finding the water! Big funding potential there, for decades to come.