Also a lot of them were shot in Italy, with Italian extras.
Fun fact: Sergio Leone would shoot his movies after the score had been composed.
He would shoot scenes with the soundtrack for the scene playing over loudspeakers.
Then, the dialogue and other sounds would be dubbed in.
This was how such perfect synchronization with the soundtrack was achieved.
EDIT 2: fun fact 2,
My favorite quote of Sergio Leone
When I was young, I believed in three things: Marxism, the redemptive power of cinema, and dynamite. Now I just believe in dynamite.