Sales and Engineering - very different competencies. Companies like IBM are NOT technology companies. They are sales-culture oriented, and their product HAPPENS TO BE technology. In fact you could argue, the only way to win big enterprise contracts in the first place, is to be a sales-culture company.
But after selling the deal, their workers have to solve a difficult engineering and organizational management problem simultaneously (the project).
They use contractors (devs) which proves they are not an engineering but sales culture. So, you need to bridge the two worlds. That's the PMs.
These are the most incompetent members of the team. The devs very often are very smart.
Draw horizontal lines on the org chart. The sales guys are at the top. They are generally competent. Proof = they sold a massive deal. The devs are at the bottom. They are often competent. They have to be or they wouldn't get hired or find work. You can PROVE someone doesn't know dev work. But, they don't have enough POWER to change things or fix things.
Reasons for failure:
1. The middle. This is the breakdown. Many PMs often have no real skills. ORGANIZING for success, given a complex technical and organizational problem.
2. The projects are too big. Any huge project is from the get-go at an unacceptable level of risk. Decentralization, Deconstruction is powerful. The projects must be broken into smaller pieces to be managed.