I think a fair amount of it is just organizational dynamics, game theory, human nature, etc.

When you build an organization around one of these sites/apps, as the product ages, each work group ends up trying to justify their existence. It's very rare that anyone periodically tries to see if the current headcount and budget for any group still makes sense, and reduces or eliminates groups as that happens. This spread and growth also means the groups all end up with different ideas of what the overall goals, measures, etc, are.

So they all grow based not on whether they need to, but because it's human nature to empire build. And to do that, you need to be "busy". That busy work eventually pushes out to prod, and ends up as more bloat. Multiply that by each group, and you get what you're seeing.