Perhaps because it's a necessary orchestral skill. They play an A and everybody tunes their instruments to it, producing that awesome wash of sound you get before a concert. For the violin you tune the A string and tune the others by hearing the open fifths between them.
For a guitar it's easier because you have frets, so you can play the fretted note on one string that should have the same pitch as the next (open) string up, then you just need to compare two pitches that should be equal, rather than hear an interval. (I'm not a guitarist so I don't know if that's considered a good technique.)
Perhaps it's just not considered such an important skill in the small groups people typically play the guitar in.
For a guitar it's easier because you have frets, so you can play the fretted note on one string that should have the same pitch as the next (open) string up, then you just need to compare two pitches that should be equal, rather than hear an interval. (I'm not a guitarist so I don't know if that's considered a good technique.)
Perhaps it's just not considered such an important skill in the small groups people typically play the guitar in.