>>2603771Short answer, Polarity response, Proximity effect and unwillingness to do overdubs.
Long answer: the wrong microphones, doing the wrong job at the wrong distance from the subjects.
A shotgun microphone (that little red spider dildo) has a very short "angle" of sound it picks up, it's very directional, it cuts out as much as possible and lets you point it a sound you want to isolate, think of it as a flashlight vs flouro tubes. In a normal "real" movie this is usually as close as possible to the subjects, pointed at their mouth and just out of frame.
With the proximity effect, their voices are (obviously) louder due to the mic being closer, but they're also bassier and "fuller" sounding, this means you can turn the gain down, which in turn makes the background noise and "room tone" quieter, which means reflections and reverb don't appear as audible in the final sound.
in most wide angle shots where you can't put a mic close to actors the sound is either dubbed over later of they have hidden lav mics, to a lot of filmmakers audio editing and dubbing like this is either too scary, or too time consuming for them to consider.
sorry if any of this seems patronizing to anyone but I'm just making sure I cover it 100%
