Because an idea is not property, it is an arrangement of neurons in your head. If one person takes a piece of property, he deprives everyone else of the use of that property... However, the same idea can exist simultaneously in every single mind in existence, and no person is deprived of the ability to hold that idea in their head by the fact that someone else is also holding that idea in their head. The natural, default state of an idea is that it is freely available to *everyone*.
But copyright doesn't (or at least shouldn't) cover ideas, it covers creative works. Millions of people could have the same idea and still no one would be producing copies of other's creative works. So copyright can't prevent anyone having ideas and producing creative works from them.
However, we recognize that certain people have ideas that most people couldn't have by themselves. In general, these ideas are useful to society, so we want these people to keep having them. In order to free these people from the distraction of having to earn a living by conventional means and allow them to spend all of their time coming up with more of their ideas, we create a mechanism by which people can profit from them... A completely artificial concept of imaginary property. We allow these people to be the only ones who may use their idea for profit for enough time to keep checks in their mailbox till they come up with their next idea. After this time passes, the idea reverts to its default state of being available to everyone.
Again, copyright shouldn't be about ideas but creative works. A creative work is neither imaginary or artificial, it's an idea realised by creative expression: the creator's work, so it is in the ownership of the creator by default. And so copyright protects the creator's property. Ideas are never denied to anyone.
The reality is, when you come up with a creative work, you do the original work of creating it *once.* Most people have to work continuously to earn a continuous living. What gives a creative person the right to earn a perpetual living off of a single act of creation?
The fact that it's the creative person's creative work. Their work, not anyone elses. Their copyright is not preventing anyone from creating their own creative works, just directly copying the creative works of others.
Imagination is more important than knowledge. -- Albert Einstein