Answer:
For years, content owners and users have been arguing about whether using intellectual property without permission was the same as using real property without permission. If I take your watch, is it the same as "taking" your song? And what does it mean to "take" a song to begin with?
A watch has intrinsic (or "real") value based on the parts and labor that went into making it; a song's value is much more subjective: it probably means more to you than it does to me. Plus, if I take your watch, you don't have that watch anymore. But if I take your song, by uploading a recording of it, photocopying the sheet music, or performing it myself, you still have something, even if it's now suddenly reduced in value.
Since I can't "take" your song, what I'm doing is taking your rights to the song. Copyright gives the creator the exclusive right to copy, alter, distribute, or perform/display the work or authorize others to do so. Even if I do "take" your song (say, by stealing a CD of it), those rights are still yours.
Ultimately, this is why copyright infringement is called infringement instead of stealing, theft, etc.: it's infringing or violating rights given to the creator under the law. Think about other ways you hear "infringement" used: infringing on privacy, infringing on right to free speech, etc. There's nothing being "stolen" in these examples. In fact the word "infringe" comes from the Latin for "break," or "weaken."