Also why are there 3 mics if it doesn't use them? Or does it use them but then mix 3 mics into 1 channel mono that seems pointless..
The three mics are used for noise cancellation. In phone-to-your-head-mode, the bottom mic is favored as the mic most likely to pick up your voice. The other two mics are used to sample the surroundings. If a sound is picked up on the bottom mic
and on one of the two other mics, then it's processed out as noise. Your voice will be picked up loudest on the bottom mic but faintly on the other two, and so it will not be filtered out.
In video/facetime mode, the same process is repeated, but the mic closest to the camera in use is given preference.
In speakerphone mode, the top mic is usually given preference (though this can change depending on orientation).
The only thing I can think of is that they want to make the file size of the video files smaller by using mono instead of stereo. But this is pointless too as recording 1080p in 60fps will make huge files anyway. Still I just don't get it.
It's probably a more simple explanation: They expect that most people won't care about stereo recording, such that it wasn't worth the effort to engineer in stereo mic pairs for each of the three points that the phone can pick up audio. You would have to do all three if you decided to go all-in with stereo, because if you engineer stereo for just one point (say, the bottom), then when you hold the camera up to take a video, you're only recording sound from one side on both channels. DO it for the camera, and then it won't work for use as a voice recorder on a flat surface. etc.
It might also interfere with the noise cancellation circuitry, and they may have felt that was more important than recording in stereo.
Also, if someone really cares enough about recording in stereo, it's figured they can just get
one of these or
one of these and deal with the positioning limitations that come with it.