Partly because the work does need to be done—especially when it's urgent—and the product manager may not believe there is anyone else available or able to do it. Partly because a lot of product managers have never been trained or coached on what product management is really all about, and they think that's what the job is. And partly because I think many people are actually more comfortable with the project management tasks, as they are tangible and much more straightforward, and it can feel productive to check lots of things off the list every day.
Good reasons why project management is what is mostly done.
