By the same reasoning Bach couldn't write a cantata/week, I suppose.

1. Carlo Rustichelli exaggerated. Morricone may have managed more than 20 films in a few of his glory years, but only a few of them required the length, the orchestral forces, and intricate dramatic connections to the films as Hollywood style score.
2. Morricone did employ a conductor for many of those films.
3. Perhaps having to work with the resources (and management!) of all those big Hollywood studios is hindrance, rather than a help.

P.S. - Max Steiner at RKO did work on more 30 films/year
(from BYU's "The Max Steiner Collection")
1929 - 2
1930 - 8
1931 - 28
1932 - 38
1933 - 41
1934 - 36