By global standards, the United States has hilariously low density and has hilariously high incomes, which means it's significantly more expensive to maintain a network. You're comparing a network that doesn't have 5G coverage in towns 20 miles away from the center of London [1] to one that has coverage over a gigantic proportion of the area that actually contains people at any given time. [2]

1: https://www.nperf.com/en/map/GB/-/164526.Vodafone-Mobile/sig...

2: https://www.nperf.com/en/map/US/-/85.T-Mobile-inc-Sprint/sig...