Managers are heavily expected to keep growing their team(s). A manager whose team is flat in terms of size and scope year over year is going to be viewed as underperforming, even if they dramatically improved what they do own (in most of the company, at least).

This means a lot of managers hire for the sake of hiring, and create projects to facilitate that hiring rather than because the projects add any real value. They'll work with their PMs to invent some numbers to sell the project to leadership, but at the end of the day a lot of what goes on there is makework, solving problems that don't exist or re-solving solved problems without improving the solution significantly.

That said, since there's so much cutthroat resource contention, plenty of extremely important/valuable projects are chronically understaffed. I'm sure there's meaningful work for most to all of the engineers currently working at Amazon, but a pretty significant chunk of them are absolutely not doing anything meaningful today.