Because we both cannot use your property at the same time (thus, if I were to use it against your will, I would deprive you of its use). We can both use your idea at the same time, so I can use it against your will without depriving you of its use. You don't seem to realize that the reason theft is bad is because it deprives the rightful owner of its use.
Think about it this way. If we could make copies of physical things as easily as we could information, there would be no need for property rights. If you had a lawnmower and I made a copy of it, have I diminished your use of the lawnmower in any way? Perhaps you sell lawnmowers... well, it's kind of dumb to try to sell copies of things that can easily be copied. Should you be able to kill me in order to stop me from copying your lawnmowers?
Intellectual property is just as stupid.
So I can move into your house, use your bed when you're not using it? Use your kitchen when you're not using it? I'm not depriving you of anything as long as I stay out of your way, correct?
I do realize theft is bad, I just extend it to "think-work" too. I do believe IP could do with some reform, but eliminating it is bad imho.
Copyrights are good for preventing you from just copying my work. You can still write your own song, book. Just don't take mine. Your work is yours, mine is mine.
Patents are good for protecting an implementation. If you can find another way of doing the same thing, go right ahead. Don't copy my way of doing it. Your work is yours, mine is mine.
Killing you? That's a little extreme don't you think? I never suggested that anyone should be killed over IP.