The early days of the Internet in the US focused on repurposing phone lines and phone line equipment and protocols. Mix that with a low density population and you get slow speeds. And few not all ISPs have been good about migrating to more modern equipment and protocols.
This stands in stark contrast where smaller and poorer nations who build their Internet infrastructure from scratch used more modern equipment and hence have faster average speeds than the US despite inventing the damn thing.
So yes, some of that is still dial-up, some of up is DSL and some of it is cable but with old ass equipment.
Which is true for basically everything. Many people complain how 3rd or at least 2nd world countries have better internet infastructure or a more modern transportation system and whatnot. But it's so obvious that this is only because the developed countries started building their infrastructure much earlier based on old technology and that many of these investments haven't payed off just yet, while costs for maintenance are high and a nation wide upgrade would cost enormous amounts of money.
And of course your point about smaller countries where it's much easier to cover as much people as possible.