Your post sounds conspiratorial but there are basic economic reasons why those revolving doors seem to exist.
Every campaign is going to need largely the same set of skill sets in their campaign staff. Spinning up these groups and also going through the startup time of any new team learning to work together costs a lot of time and money.
So several of these standard skill sets, like data science, marketing, etc have been spun out into companies or consulting firms that are treated like a pool of available resources by campaigns based on their party.
It’s not treated as a slush fund and there’s usually a handful of competitors in your parties pool but you do end up working with a lot of the same faces at different clients/campaigns if you work at one of the servicing companies.
I worked at one of them once and I recall realizing that fact when I asked why a coworkers email had numeral in his name when he had a relatively uncommon first and last name
