Because the voicings change quite a bit with what you're able to reach in different tunings.

I'm not saying alternate tunings are mandatory, just that you can't learn a single tuning with guitar and expect to only ever know just that. It's Very common to change tunings in guitar. Not so in Piano, that i've seen yet at least.

> Piano is also a stringed instrument, with one common tuning that most of us westerners play in.

Are there different tunings for Pianos? I'm not even sure what different tunings would look like, non-sequential pitch ordering? C next to G or something?

The only Piano "tuning" i'm familiar with is temperament, however that's functionally different than what we're talking about with Guitar.