It really is what you get used to. Learn on. It’s all about muscle memory.For me it’s the piano the more difficult. I can’t do two different things at the same time. That’s how I see the piano. Another example is that I can’t play bass and sing. That’s two different lines in my head.
For me on guitar, the left and right hand work in unison. I can strum and sing. Certainly can’t lead and sing.