Because they all contributed ideas, and if, say, Paul had the best idea for a lead guitar hook or solo, he'd simply play it rather than take the time to teach it to George Harrison. I like the story about "Obladi, Oblada"-- I'd always assumed that Paul played the piano part, but it was John. Apparently, the arrangement was lifeless until John walked in, heard a bit, then sat right down at the piano and played the intro. John also played bass on "The Long and Winding Road", George played bass on "Old Brown Shoe", and Paul played drums on "Dear Prudence", "The Ballad of John and Yoko", and a few others. Ringo even played Hammond organ on a song on "Rubber Soul" (I can't remember which at the moment).