In some places they are, but also there are immense swaths of green rolling hills that stretch to the horizon on all sides. [1]
Speaking to locals they didn't think "modern" humans had deforested like that - the evidence would have been visible, and everyone agreed it would have been impossible to deforest that much landscape without at least steam powered tools.
Maybe fire explains some of it, but I still don't really understand.
Amazingly, when I was on the other side of the continent a stone's throw from the Eastern side of the DRC, the landscape was virtually identical again. [2]
[1] Here are some shots from DRC, for example: http://theroadchoseme.com/across-the-drc-day-2
Video of each country too:
Gabon: https://www.youtube.com/watch?v=GAglvUMRj3c
Rep. Congo: https://www.youtube.com/watch?v=rOn47q06RRM
DRC: https://www.youtube.com/watch?v=OV8V3GdOcPU
[2] Uganda (almost Eastern DRC) photos: http://theroadchoseme.com/ishasha-national-park