What about shady services that won't unsubscribe your email even if you do it from their admin panel or just outright doesn't work? What about services that provide a really complicated unsubscribtion procedure, e.g. enter password, fill multi-page quiz why you are leaving, wait up to 24h for some code that you will have no idea where to enter. And finally, I suspect there are just plain evil "services" that after unsubscribing will put up your email on various spam lists - have fun with even more spam.
Good behaving email marketing services are not the problem, I would gladly pay for a service that deals with those shady ones and maybe even helps to battle spambots on a global scale, by reporting spam to appropriate authorities and even using GDPR laws to stop them. Tried using spamcop once, but most reports end up "nulled" and not actually reported.