Let’s think of a simple example: you’re starting a new project, the current infrastructure is technology A, but one engineer proposes technology B, since it’s better in categories X,Y,Z. You can plug in anything you want here. Now, the engineers can give their opinions and estimates, but they can’t decide it. The PM can. It’s his job to weigh the risks and uncertainties and decide on the path forward.
Again, as the guy above, you’re thinking way too narrowly focused on your small slice of the world. IT departments aren’t magical omnipotent collections of super smart people, revolving mainly around themselves and their superior technology. They’re just cogs, we are cogs, and our job is, to keep a machine running.