I mean, it's kind of an insurmountable obstacle. Why bother trying to unsubscribe when you're always gonna get spam anyway? It's just gonna come back.
Also, websites are shady. If you put in a required email, they'll usually automatically check a little box for you that says "allow us to ruin your inbox?" How helpful of them.
And, I'm not even convinced that checkbox does anything.
It's not insurmountable. Spam filters have been around for decades. They're pretty good. If I didn't expect the email, I train my spam filter that it's a bad email. There are a few that get through, maybe 1-5 a week, which require flagging.
The checkboxes seem to be a placebo. Sometimes there isn't one. Sometimes it doesn't do anything. Sometimes they say "updates on your order", which apparently also means future products you might want to buy a week after you receive your order. (Marketers' English seems like a foreign language to me).
I just flag every marketing email as spam. It's much more effective than unsubscribing since it tells your email provider to just redirect everything from them to spam, and it causes trouble for the sender.
It's definitely not insurmountable. It just takes a little bit of inbox maintenance. Pressing unsubscribe and report when I have spam in the inbox, 5 seconds here, 5 seconds there. I still get spam, but it's minuscule compared to not doing this.