Totally speculating, but from the outside it seems like stagnation & resistance to change. The Corvette still uses leaf springs. All three of the Big Three hung on to SOHC & other relics for far longer than Japan. I also remember as a shade-tree mechanic, Japanese engines were always precise, intricate, and sharp (suggesting computer design & milling) while the Fords of the same decade were not, and frequently the arrangements were irrational. Rangers needed the intake manifold removed to access the spark plugs. This always suggested to me more casting & hand drawing.

American automakers seem to make some great stuff when they are actually pushing the envelope- the EcoBoost line I hear is very solid, and the aluminum frame in the F150 seems to be doing great (notice: lots of FUD when it launched, silence ever since). The Volt & Bolt have had many great things said about them.

Whether it's conservative engineers, or tight purses, or unrealistic schedules, or even just myopic American Exceptionalism, I do not know.

Corvette uses a tranverse leaf spring, which isn't the same as most people expect. Chevy does it because it works well (it's a top handling car, held the lap record at Nürburgring for a while) and allows for extra cargo space in the rear.

IMO it's a combination of multiple factors. Chasing short term profits definitely plays a role, but there are other cultural factors. An adversarial relationship with labor[1]. Over-reliance on inspection processes to catch production issues. Poor engineering culture that focuses on individual issues as they occur instead of taking a systems-level view. Over-reliance on metrics and KPIs. Performance review criteria that incentivize ignoring or under-reporting issues[2].

Also note that fuel pumps may not be the best example. American manufacturing of mechanical components has gotten a lot better over the past couple of decades. But electronic components are the new opportunity to repeat the mistakes of the pasts.

[1] Watch https://youtu.be/qg8bbFwZLMA?t=1760 through about 44:20, which is an decent overview (although the explanation of Kaizen focuses too much on efficiency). Compare the description of the Japanese relationship with labor to how labor is usually treated in the US. Also notice from 36:40 on how much focus is put on technology versus process (although that may be forgiven since it is a TV show).

[2] For example, in theory you may say your employees have the authority to stop production if they notice an issue. But if the line workers are graded on how many parts they produce, and the engineers are graded on how much downtime there is, nobody is going to report anything.

The US stock market? How do you plan for purchase horizons of ten to fifteen years and very little maintenance operations in that window, when your shareholders only care about what you did for them for a quarter of a year at a time?