Is this sarcasm? 90% of rock songs are in Em or Am because of guitars.
What I meant is that playing a C major scale is easy, but it gets weird as you work through different keys. There's no consistency, so transposing is non-trivial. Compare that to a guitar and barre chords and capos.
Transposing is harder, but the vast majority of piano litature is never played tranposed--you play the notes on the sheet music. I've tried some of those new layout keyboards like the Linnstrument, and I'd never try to play a Beethoven sonata on one.
This actually presents a hurdle for me, because if I learned a theme in G, and now I have to play it in D I feel like I know it, so I don't practice it as much as I did initially and it ends up in this weird limbo where I know what I'm supposed to play, but haven't properly trained my fingers.
But there's plenty of stories of Liszt or Chopin transposing music on the spot. Jazz too.
In guitar transposition is easy conceptually, but not so much technically, not being able to rely on open strings as much is a big limiter to novice guitarists. And guitar being such a popular instrument, there's a lot of those.
On average technical limitations will dictate keys more than conceptual limitations.