Piano is a core instrument, all my music teachers have played the piano. Can you think of a famous guitarist like Beethoven or Mozart? Piano has every existing note on it, it is capable of much more than the guitar.The majority of musical masterpieces cannot be played on the guitar. Can you trill on the guitar? Can you play flight of the bumble bee on the guitar?
Pianists have a lot more skills than guitarists. Pianists fingers are more agile and whereas a guitars stings are placed so conveniently for guitarists to pluck or strum. When a person sits down in front of a guitar, he/she could probably strum it and make a good sound easily. Whereas if you placed a person in front of a piano, it would sound like a mess. Which just comes to show how complicated piano is compared to guitar. But the many keys of the piano allows for a pianist to get a wider range of sound compared to the guitar.
Also the guitar is limited in what it can play. At the most it can play six notes if you include the thumb unlike a piano which can play ten notes and more if you add the pedal. This means that a guitar can only play chord or the voice whereas the piano can play both.
Guitar solos are great but they only sound great because of the mass amounts of electronics guitarist use to make them sound good. Put a great guitar solo on an acoustic guitar and it just sounds lousy. A piano does not need any sort of electronic addition to make it sound great. Sure, a keyboard is an electronic but any professional pianist knows that a keyboard is only used because it's portable. Get grand piano, an acoustic guitar and professionals who know how to play each instrument. Even though the guitarist is good, he/she will never come to par with the sound that the pianist can make. And if you really insisted on wanting to put the electric guitar in the equation it still wouldn't tip the scale. Any pianist could play up to par with an electric guitar. In fact they could play the exact same by playing on a keyboard in the electric guitar mode.
Finally piano's sound better than a guitar. Who seriously wants to listen to the chords of a song on the guitar when you can listen to the the voice and chord on the piano? Piano sounds amazing and if you're an expert then you probably have a couple of pieces (songs) up your sleeve that can impress everyone and make them go wow. A guitarist can have that too but usually they're guitar solos. And their guitar solos probably require an electric guitar, a speaker, all the pedals and assortments you need to make your guitar solo and all the wires to plug in.
The piano is a sophisticated and elegant instrument which sounds beautiful when played. The guitar is great and it does have it's own specialties but when compared with the piano it lacks the great complexity that allows the piano to play such elegant songs.
If you really doubting me check out these videos.
Can a guitar play that? Now watch this one.
This proves that the piano can play the same music as guitars. (Note the speaker and all the wires?)
You said that pianists have a lot more skill but that's not true. Guitarists fingers are more agile and a lot more stronger than a pianists. Plus guitar solos are amazing. There's a lot of things that guitarists can do that pianists can't and to be quite honest guitars sound a lot better than pianos. Pianos have a dull plinking sound that's annoying. Plus beginner pianists sound horrible and pianists always play that annoying song called Fur Elise because they think it will impress people but we've heard it too many times and now its annoying. GUITARS ARE BETTER THAN PIANOS