Almost all cinema movies are 'dubbed' - ie the actors recreate their lines in a post-production studio using Looping. The Looping process (the actor sees a short scene - usually a single line of dialogue - that loops while they rehearse their line until, when the rhythm and intonation are what the director wants, a take is recorded. Then on to the next line.

The process emerged from the problem of on set/location sound recording quality due to the cumbersome technology of earlier years. But today the problem is noise pollution. It is hard to record good sound on location today - too much extraneous noise.

Of course some directors use location sound, using methods developed for documentaries. But sound is such an important part of movies that its reconstruction - dialogue, FX, Foley etc etc is considered to be a creative process, not a budget process.

Editing of picture, as well as dialogue performance, is another reason for (re)dubbing. When you cut pictures together you create a spatial relationship between the shots. The associated sound has to fit into the space. It is a lot easier to do this afterwards in dubbing - you start with a 'spaceless' voice recording and then add the echo, EQ and ambience that place the voice in the picture space.

Leone, and many other Italian directors, recorded the most primitive of guide tracks on set, knowing that they would recreate the sound in the controlled environment of the studio. Leone took it one step further in often choosing non-actors for the look of their face, and getting in a professional later to 'do' their voice. He also took a lot of liberties in changing the original dialogue. And that led to strange dubbing artefacts - noticeable in Once upon a time in the West.

You can't be me, I'm taken