I think there's also an element of the scientific bias for assuming the mundane leaking into the public. Which is fine, but scientists have learned to temper that by maintaining a strong sense of curiosity and determination.

Personally, my gut instinct is that the chances of life having existed on Mars at some point are quite high, above 90%. But the evidence for such may be very hard to come by. What if all life on Mars had never advanced beyond a unicellular level? What if most/all life died out one or two billion years ago? How challenging will it be to find evidence of that life? Especially when we only have a few robotic explorers to work with?