The thing is this - every year we send out search teams for lost hikers who presumedly want to be located, we know their whereabouts, yet we find nothing. Couple that with the idea that there are fewer people in the bush now as opposed to just a few hundred years ago and it wouldn't surprise me that we can't find a sparse population of intelligent animals that don't want to be found. I've spent hundreds of hours in the bush just an hour outside of Calgary and you can go all day without seeing anyone else. We're not exactly a hunter gatherer society anymore and most people don't go 10 from a paved road.
No, the Congo isn't America, but it does have some challenging geography and isn't a densely populated as Europe. Scientists didn't believe Congo locals just as they don't listen to the First Nation's folks who think Sasquatch is real.
Does that mean there's a Bigfoot? I want to believe, but just don't.