By default they do block quite a bit. The "Standard" tracking protection option in their Settings page says it blocks Social media trackers, Cross-site cookies in all windows, tracking content in private windows, cryptominers, and fingerprinters. They have a strict option with a disclaimer that it may break some sites or content that does a bit more.

So they're already blocking as much as they reasonably can without affecting legitimate functionality, and they have an option to block even more.

As for "Why offer them anything?", my guess is pragmatism. They're a lot more likely to succeed if they propose a system where the users give up nothing but companies can thrive anyways, vs. a system where the users give up nothing and the companies in charge of everything just burn to the ground and die.

I notably don't have a strong opinion on whether or not I think they'll succeed with this feature. I think their intentions are pure, though, and that it legitimately offers no privacy risk to users at all. I think the best chance it has is something like government mandates. Maybe there's also a future where they somehow get Google on board for PR reasons or something. I wish them the best of luck.