Good point. That's a good way to put it. Depending on task, unfortunately not everything is parallelizable.
But quite often, culture is the problem as well. If performance bonuses are large enough, people will compete with each other and will be at each other's throats over control of projects. Machismo and cowboy-like behavior is to be expected in some environments. I personally think those environments are toxic.
Anyway I see the a point about some tasks not being easily de-composible, but I think culture plays a significant role here as well.