Why is it that right-handed people play stringed instruments such that the strings are actuated with the right hand (finger picking, flatpicking, strumming, bowing), and fingered with the left? Many left handed players reverse the arrangement. On the other hand (pun inteneded) left-handed pianos are almost unheard of.
On the guitar, the left hand is doing a lot more than just pressing on strings: chords, slides, bends, tapping as in hammer-ons/pull-offs, natural harmonics, even muting strings in some techniques (see Stevie Ray Vaughan’s style).
The right hand is also doing more than just strumming or picking: two-handed tapping, artificial harmonics, palm muting, using the whammy bar to produce vibrato or whole-chord bends.
There are also techniques like sweep picking which require very precise coordination between both hands.
Same goes for fretless string instruments: playing long notes by drawing the bow over the strings (legato) is the bread and butter of bowed instruments like the violin and the cello, but they can also be played with short stabs of the bow (staccato) or by picking the strings with the fingers of the right hand (pizzicato), among others. Vibrato and slides are played using the left hand, simply by moving the finger up and down the string.