I had my identity stolen and after a lot of research I determined they were able to gather the info from all of these public sources. I have been trying to scrub myself from them for 3 years now. It's like playing whack-a-mole. Some are quick to delete, only to pop right back up again months later. Some just don't respond. Some require physical mailed requests. A few are openly hostile. You need to submit forms with google to get search results cache updated for each page.
You also need to clear "associates" such as spouse, parents, children because your info appears on their profiles as well. Tedious process!
I believe the best defense is to have a common name or create a ton of spoof info to confuse them.
I wouldn't be surprised if a lot of it comes from customer lists (either sold directly by the collecting company for a little extra profit or bought in bankruptcy/liquidation proceedings).
Then closely monitor. Submit search results cache removal as soon as it changes (https://www.google.com/webmasters/tools/removals). I've found it's best to do one giant wave of removals to avoid propagating from one site to another.
https://github.com/yaelwrites/Big-Ass-Data-Broker-Opt-Out-Li...
I believe it is widespread today at least in several countries their intelligence really have a kind of "global human database" where everyone is tracked and all digital traces are carefully collected and can be used at appropriate moments for different interventions (like elections, policy change, trade agreements, etc.)
I totally agree with you about how unreasonably burdensome the process is (it probably took at least a solid 40 hours of effort for me to get my results mostly scrubbed from publicly accessible sites a few years ago).
However, I think you have to put an asterisk next to "opt-out." In a lot of cases, your data might still be accessible in premium or specialized products (like those for law enforcement). I know of no law besides the California-specific CCPA that actually regulates opting-out, and a lot of these sites like like they're operated by sketchy people.