The problem I've seen is analogous to building skyscrapers and garden sheds. Some projects, like skyscrapers, have be complely designed in advance and built to be a skyscraper. You don't build a 2-story building then decide it works pretty well and then add another floor, and so on, until you have a skyscraper.
At the other end, you want a shed to protect your tools from the weather. While you could design a shed and the process to build it the way you would a skyscraper, but that's a lot of expense and it's going to take a long time. Instead, if you wanted, you could throw up some supports and put up a roof. This gazebo works well and protects your stuff from the rain. It's pretty easy to add walls, and so on.
Agile is useful for certain kinds of projects and the "classic" way is good for other kinds of projects. The real problem is that people try to use the same methodology for every project.