Well, they sound less dissonant for the reason that you said - that the frequencies of the harmonics in the group of notes sounding simultaneously clash less. This graph from https://sethares.engr.wisc.edu/consemi.html shows how pairs of sinewaves less than a minor third apart give an impression of 'roughness':
However, having notes further apart doesn't always sound subjectively better. One reason for that is that although you're reducing the potential for harmonics to clash, you're also reducing the potential for the strong harmonics to coincide, which is part of what can make chords sound so nice - the way that they sound like a single sound, rather than just a group of individual notes.
Another reason that having notes further apart doesn't always sound better is that dissonance can be a desirable thing. modern r'n'b songs often feature close-voiced extended chords played on electric piano, which have a characteristic biting quality at the start of the chord, and then mellow out as the higher harmonics die away (yet still leaving some throbbing beat frequencies for interest).
...Which gives us another thing to consider - the instrument being used to play the chords. Different timbres will cause different clashes and correspondences between the harmonics in the notes being played. This in turn ties in with the way that close voicings in bass registers can sound very 'muddy', while close voicings in mid-range registers can sound sweeter.
Additionally, with stretch-tuned instruments such as the piano, two notes that are a long way apart could be quite a long way out of tune with each other.
Of course this is all subjective and any given listener is free to say that they do always prefer more open voicings!