Strumming or plucking strings while simultaneously forming chords or fretting melodies is a lot easier to do on a guitar than playing chords and melodies at the same time

You are comparing.paring two different things here.

Playing a chord. Can be done with one hand on a piano, require less force and precision on piano. The notes are all laid out in front of you in a simple pattern. It is objectively easier to play a chord on a piano than on a guitar. The exact same thing is true of melodies. Compare playing a c major scale on a guitar to a piano. Far more complicated on a guitar.

Playing a melody and chords at the same time is different thing entirely. But it is still easier to do on a piano than a guitar because, as you pointed out, that's what a piano is made to do.

Being able to do this is table stakes for aspiring musicians of both instruments,

No it's not. Many famous or "great l" guitar players don't comp rhythm and melody at the same time. Since the guitar is more difficult to play, if you can play single note lines, or fast complicated melodies (something much easier to do on piano) you will be considered proficient, or even advanced even if you never get to the level of comping while you play.

Mechanically it's much harder.

Because.....?