Multiple reasons. In addition to the recording equalization and downmixing of multiple channels to stereo mentioned in other posts, there is also a problem with the actors just not enunciating their lines clearly enough. Either they are going for method acting feel, or they are just bad at acting.
Acting used to be done on stage, body languages and clear enunciation is critical to present the role to the audience, especially when sound amplification did or still does not exist on stage.
This kind of acting tend to appear to be over the top in films, so actors tend to whisper at normal volumes and their voices are drowned in the noises generated by camera and lightning equipment. Post processing or voiceover is required to restore the audio. If that isn’t done correctly you still can’t hear them.
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