Easier to learn, harder to master.
It's much easier to play chords on the guitar than to play them on bass, so it's easier to create a melody on the guitar.
The bass also has wider frets and fatter strings, so you have to press and hit the strings harder (sorry for bad English).
Then there's 5,6,8,12 string basses, fretless basses etc. if you think that a 4 string is too easy.
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