I've been a jazz saxophonist for 35 years, and am recently self-taught on jazz guitar. I mainly play chord-melody, but also do some single-note soloing as well. I tend to doubt the accuracy of some of the other posts here, but I can tell you with good authority the reason jazz guitarists in general don't bend notes (I'm talking about real, straight ahead jazz, mainly bop and swing, not fusion, blues, "smooth jazz" or other rock/pop influenced and commercially watered-down varieties).
Here it is: The modern jazz guitar solo style started with Charlie Christian in 1939, and the breakthrough that he made was to use the amplified guitar to play horn-like lines so the guitar could stand on it's own (volume-wise) as a solo instrument alongside the sax and trumpet front line, something that had never been done before. Unlike the (mainly) chorded, unamplified guitar solos that occurred in jazz up until then, Christian played long, looping boppish single-note lines in the manner of a saxophone. This adaptation of the guitar to emulate horns did not include bends simply because most horn players did not (and still do not, especially modern players) bend notes. Although it's easy to do on a guitar, it's much harder on a horn, especially a sax. (Besides this, the innovations of bebop included playing very FAST and clean. You can't play fast flurries of notes and have time to bend anything, even if you wanted to!)
The jazz guitarists who followed Christian (virtually all of them) continued this practice of trying to sound like a sax-- clean and precise with no bends. I've never heard Joe Pass bend a note! This continued to the current day.
Another poster here mentioned that jazz guitarists tend to use heavier gauge strings, which are harder to bend. Heavier gauge strings give a louder acoustic sound, provide a richer, fuller tone, are easier to intonate and stay in tune longer, but have nothing to do with jazz players not bending notes. Once the electric guitar became established as a viable jazz solo instrument (early-mid 1940's), it worked its way into blues, r&b and later rock & roll, and THOSE players expanded the amplified guitar's sound palette to include not only bends, but distortion and other sound effects achieved not only with the guitars themselves, but by use of electronic gadgets.
So while blues, r&b and rock guitarists bend, distort and use all kinds of echo, chorus, flange, fuzz and other effects (to the point where the original timbre of the instrument is all but completely obliterated), pure jazz players like to stick to the original formula of keeping it clean, straight and no frills, so it still sounds like a guitar! In fact, the best jazz guitars have carved tops, which allows the instrument to vibrate freely and thus add a measure of acoustical warmth to accompany the sound of the electronic pickup.
