I'm betting that doesn't work for every country in the world with unlimited data. If it did, I'd like to hear the carrier that pulled this off and the price of the service.
T-Mobile and Google both do it in 215+ countries for unlimited basic data (not 5G). T-Mo charges between $75/mo for that and $90/mo for 5G data internationally (not unlimited). Google charges $35/mo with unlimited data (doesn't guarantee 5G). It's not difficult for them to do or even expensive. Most just choose to make it more expensive.
Well it's still a lot more expensive than the $5-$10 I pay on an ad hoc basis for an eSIM when I need one every few months, even if I was traveling almost exclusively 100% in countries where I needed non eu data packages it probably wouldn't pay off, but it's good to know it's out there. I guess if I was in that situation it would probably be worth it just not to think about it (at least the Google price would be).