ethernal

Do you have any data to support this? Pre-COVID, the US was filled with congested airports, jammed up airways (especially East Coast North-South routes), and unpredictable weather. You can certainly find global airlines that have better on-time rates but they are always based in uncongested regions and/or lovely weather (just like how Hawaiian Airlines almost always topped the charts in the US - despite being nowhere near as operationally mature as the majors). In addition, many other famous global airlines are single hub airlines. Constant out and back routes to mega hubs are operationally easier than complex meshed routing that bounces around hubs throughout the day. And if you are international-biased, many times there are long turn times that act as buffers to flight delays that don't work on short turn hauls.

Delta pre-COVID (and United was getting there) were also very good at not cancelling flights, especially when comparing like-for-like operational environments. You can argue about the definition (is a 20+ hour delay a cancel?) but few other airlines were as aggressive and creative at avoiding cancellations and/or started replicating tactics pioneered by US airlines (like enroute fresh crew pickups).

This isn't to say US airlines are great (or not great), but trying to compare them to less operationally complex airlines or those that operate in more favorable environments is silly. You can rag all you want on hard and soft product differences, but I don't think there is much of a leg to stand on when it comes to operational reliability.