The strings sound quite thin - they have that twangy feel which you get if you string a steel acoustic with very light strings.
Also some of the guitars look quite small meaning a short neck, which means the strings can be looser to get the same note. The looseness adds to the effect Tetsujin mentions :
When you hit the string to get volume, for a moment it stretches while it's vibrating a lot (a bit like bending the string very slightly) and the note is slightly sharpened until the string settles down and gives a more reliable note. It makes things sound slightly out of tune.
The third example (possibly others too) has 'wow' on the recording. This is usually caused where the spindle which drives the tape feed in the recorder is very slightly not perfectly circlular meaning the tape just marginally runs fast-slow-fast, usually about 4 or 5 times a second, as the high spot on the spindle hits the tape.
The end result if that the recording (or maybe just playback) ends up with a vibrato effect because the speed of the tape is varying. Wow is a slow variation - if it gets faster, it's called 'flutter'.
It's a subtle effect and something we're not used to these days because digital media has eliminated it completely. It's more noticable on piano music.
On a guitar, sometimes people play with a bit of vibrato, and often use it vocally, and maybe your mind puts it down to that. There's no "whammy bar" on a piano (there's an idea..) so the notes are very steady, making it much more noticable if they do waver owing to 'wow'.
The 3rd example also seems to have something which is just plain out of tune, though.