I do the same. Gmail gives me a single, standardized interface for opting out of emails: mark it as spam. All the various companies I've given my email to, on the other hand, give me different, either clunky or often outright broken interfaces for opting out. There's no direct financial incentive for them to invest in making ethical, robust opt-out systems.

However well meaning, collectively all those companies are still just a bunch of sociopaths. This might be a bit dark, but I think a reasonable real world analogy here is stalkers and restraining orders. A stalker isn't motivated to listen to you when you tell them to stop talking to you. That's why you get the restraining order.