They tend to be people who couldn't cut it as actual engineers, and so they end up in administrative roles, the ones that OP lambasted as writing stories to write stories, and causing more work, because they need things explained to them slowly and with crayons, and that takes more time, because the engineers just want to be left alone meanwhile the PM gets yelled at by customers, and decides to side with the customers and is just one more person yelling at the devs.
I don't doubt your experience but it's also a harsh generalization.
I've worked with many TPM's who were skilled engineers. It can be a path to transition to management and test the waters before taking on formal management responsibility. Or some people just think differently and are good at managing organizational complexity and finding the actual needs before starting to build things.