Not sure that is 100% correct.
There are for example Luthansa flights that make two US stops, but the 'intermediate' stop is only for passengers getting off on the way into the US, or getting on, on the way out.

Years ago, yes they made more stops and people got on and off at will it appeared. I assumed they stopped that for security but I guess the non-competition makes sense also.

As well as the need to re-fuel not being an issue nowadays (doesn't a 747 hold enough fuel for 15,000 miles?) I reckon there would also be issues with working hours.
Ive had occasional hassle where a plane from here was delayed 3 hours and we were told we had to wait for a new crew because the original one would have exceeded the number of hours they are allowed to work per 24 by the time we arrived. If they made stops they would have to change crew all the time. I bet that 20 years ago no-one cared about the number of hours.