And it takes forever to chop jungle, plus you need a level-2 tech (Bronze working) to chop, plus chopping yields no hammers (unless forests), plus if you chop you are forsaking the later science yield from jungle (post universities). Left in place, jungles are great for defense and great for science, but so-so for food (tiles are just self-sustaining on food) and even worse for production. In sum, jungles mean a slow start--not a bad start, just a really slow start.