Mostly its symbiosis. When you have that much waste in the management layers, there is high degree of mutual political co-operation. This is even more easy in a 'Product Manager' position, you have quite literally no deliverables and almost nothing to report to your immediate manager(Who themselves have no deliverables). Often the thing they have to show are some metrics, which are generated by engineers, even the information that goes into the slides comes from engineers. So they basically 'own' the product, in the sense they get to make go-nogo calls during product launches.
Product managers are also insanely famous for killing features just to assert authority(This apparently comes while trying to appear like Steve Jobs. Steve Jobs said no to features, so we say no.).
The people managers don't put up a fight because they too are probably over staffed. The Managers/Real-Contributor ratio is high, and they know if they open their mouths and protest, their jobs are on the line next. So they let things go on.
A while back 'Program Managers' were in a similar state in most companies. I know places where they fired the entire Program management org and didn't notice any change, not even a faint pain. That was how contribution-less the whole hierarchy was.
The other part is obvious. These companies are full of VC money. They didn't earn it. The founders didn't earn it. Its free. After all its not like a loan you pay back, if the company falters, the founders or anybody of authority doesn't owe a dime to the investors. So they splurge. They spend on people. Most importantly they spend on their friends, who can help them get jobs else where.
All people are more susceptible to charismatic others than we'd like to admit I think.
People will tend to believe and trust them more, and so they can spin the narrative however they'd like (and often they're the layer of separation between devs and the higher-level management that precludes the higher-ups even getting first-hand exposure to some issues).
Edit: I also think charismatic others can do this without always knowing it i.e. in a self-interest sort of sense.
