Most of them are much smaller, meaning there is significantly less distance to cover. Of course you can get fiber to the home every where when you have a small nation. Let's look at larger nations:

Canada has worse broadband but better mobile [0][1]. I'd attribute this to population density: most of their population is fairly close to our border because the rest is a frozen wasteland.

Russia has worse speeds by far, and the certain things simply are not accessible due to being blocked.

No, we're not South Korea, but S. Korea is small with most people living in a very concentrated area. Again, no other nation has companies with excellent chances of setting up satellite network infrastructure, which is a good solution for a nation as big and spread-out as America.

[0] https://www.forbes.com/sites/kevinmurnane/2016/09/15/compari...

[1] https://en.wikipedia.org/wiki/List_of_countries_by_Internet_...