They don't have to, is the thing. If Facebook can't operate as a business in their country, then they can't collect advertising revenue there, can't run FB marketplace, etc.

Facebook has always had the option to not operate as a business in a country, that they do is because it's profitable to do so.

It's worth noting that not paying fines in one country is a serious enough deal for the entire European Economic Area that it can escalate to that company not being allowed to do any trade in the entire EEC.

And this fine will increase if Facebook dont comply, it's been done before to even bigger targets like Microsoft(back when windows/office was still relevant) so just being big wont save Facebook here.