If I buy a microphone I can borrow it and give it to whoever I want to.
In fact I can chip in and share it with 200 friends if I want to.
You see the difference?
I do, do you? Microphone sharing (a) means only one person at a time can access it and (b) it is way more inconvenient. Microphone creators are not threatened by sharing, digital creators are.
If someone steals 20 microphones from the mic creator he has 20 microphones less. If someone downloads 20 songs from a song creator the creator of that song still has the song.
That is the difference.
..and so I have to buy a new one if we both want to have one at the same time.
"If I copy a song from you, you still have the song yourself."
Awesome, we can just buy one then and split the cost! But why stop there? Let's also give a copy to Tom, Dick and Harry so that each pays 20%! Now, what if we could somehow share the song and the cost with a hundred million people across the world..