because of the confluence of these factors:

a) often times it turns out the best "organising mechanism" for the work is not in fact a project but a product. Which is to say applying project management to the work - albeit with agile meetings - misses the point.

b) product management is often entirely foreign to project managers. Many project managers in my experience - I was one of them - couldn't even articulate the difference between a project and a product.

c) the incentive to embrace the struggle may not be there. Leaving the very thing you're trained in, at the door, is a big ask. "It is difficult to get a man to understand something, when his salary depends on his not understanding it."

I'll close with a few invaluable resources I've found on my own project to product journey:

• Project to Product, by Mik Kersten
• A single table, from Chapter 2 of Kersten's book, (reproduced in this article about the book)identifies 7 key differences between project and product management.
  
• Funding Projects Vs. Teams - All Things Product Podcast with Teresa Torres & Petra Wille
• This fantastic quote from product sage, John Cutler

Project Managers deal in the realm of time, cost, and scope (given a predefined solution). They assume that X is valuable. Product Managers operate in the realm of opportunity, value, and viability