They are smart. Talk to anyone who hasn't simply had a sighting, but has actually interacted with one, and the majority will agree. A bigfoot hanging out in any populated area likely knows the terrain outside every fence line better than any human in the area and knows the concealed corridors. And it's not that we're dumb, we're just oblivious.
Deer are undeniably dumb, and we see deer in urban areas all the time, but there are times we don't see any and we wonder where the heck they all go when we're not seeing them. Cougars are smarter. In urban areas where we find cougars, often the cougar has been around for a while, and very few people have a clue. It isn't until one is found that people start musing about whether or not there's a connection to all the recent pet disappearances. There's a community in Cincinnati right now where they're finding cougar tracks on a regular basis, but nobody's actually seen the cougar yet.
Bigfoot have an even greater advantage. Their intelligence range overlaps our own and they are vigilant compared to our keep your head down and get to work culture. Most people see a humanoid silhouette in the shadows and avoid it, because they assume its either a panhandler or trouble. If someone does get a clear view of one at a distance, they simply assume that it is a man viewed disproportionately or wearing bulky clothes (like some homeless people). If they get a clear view of one at close range and talk about it, they're derided and laughed at.
Derision perpetuates ignorance.