Guitars being out of tune happens because of the frets, not some lack thereof; most people tune each string with a tuner and/or by comparing to other strings, which means that some notes will be in tune and others will be very out of tune. This is one of many possible motivations (arguably the primary one besides ease of fingering) for tunings other than the "standard" EADGBE tuning: to change which subset of possible chords are in tune.

This is arguably a non-issue for fretless guitars, but those take more skill to play (and I'd imagine getting some chords perfectly in tune would entail nigh-impossible hand contortions even without there being frets involved).