>>149299577 (OP)Generally it's because a lot of American cartoonists got to have a chance to wind down, both day to day and towards the end of their lives. Many artists could move into a writing or editorial or management position. Or in harsher cases, forcibly retired by lacking work.
Strange thing is, while American comic workloads tend to be less per month, never the less they were fairly significant with 2-4 pages a day being common before the 80s, and while assistants weren't unheard of they tended to be more a guy looking up references or filling in blacks, not a crew like better off mangaka get.
someone like Curt Swan or Jim Aparo were doing 2-3 pages a day with no assistants,for example
Carl Barks and Dick Sprang(of Batman fame) both had a chance to retire in the 60's or so and spend the next few decades painting.
It's not so cut and dried though. Lately we've had a run of American artists and creators who have died under 75, and a lot under 70 even. Retirement conditions for comic artists have become considerably harder(as it's becoming for many Americans)
Shigeru Mizuki, creator of Gegege no Kitaro, mused about the phenomenon where his contemporaries would talk about how hard they'd work, and they all died at 60, where as Mizuki was alive and eating McDonalds up until a few years ago.