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This is partially due to (current) landing site requirements. Up till now landers targeting mars could not really handle any bigger terrain obstacles (big rocks, steep slopes, etc.) so the landing sites have been meticulously selected to be as flat and rock free as possible.

That's why photos from different Mars landers look often very similar - the regions they have landed in have been effectively selected to be similar. :) And of course these pictures shape how Martian surface looks like the public consciousness.

Things may be changing though, with the technology being developed for active obstacle avoidance, that might enable martian landers to land in a more complex terrain safely by analyzing the actual landing site and landing on a safe spot.

Still, you might still want some flat area near an early martian base/city, even just to make trucking stuff from/to landers that strayed of course during landing easier. Eq. not having one in the middle of a rock field and another on crater bottom. :)