[Jdanz]

because you don't own the materials.

edit: cause that's sort of a jackass answer and i'll explain my thinking a little further. Your idea implies that there is some sort of original ownership of materials, which is frankly pretty silly. There is an entire market system that relies on inertia as to who owns what, but the orignial distribution follows no principal of "justice" or even "natural law".

The concept of ownership is a pretty arbitrary one in the legal sense. Most economist define ownership as the ability to profit from a(n) item/resource/attribute/etc. Ability to exclude others from using your property enhances your economic ownership as you can now profit by renting/trading, however economic ownership is never clearly defined over ANY object, and is rather a combination of ability to enforce and societal norms.

The idea that today's (or any days) property structure is seen as/or will be seen as legitamate is ridiculous. We all inherit the world the way it was passed down to us, but that's just the way it is, it doesn't convey any legitamcy on that strucutre.

There have been countless instances throughout history of people making claims on property and ignoring other's claims on property. Insomuch as a system of property allocation doesn't seem adequate to a large enough segment of the populace, they'll simply ignore it (revolution).

The reason why a person owns the rights to their labor is that they actual control such rights, and can act or not act. The reason why they don't own property in the same manner is that they only control the use of that property so far as the rest of the world recognizes the legitamacy of that ownership. It cannot nor will not ever have anywhere near the validity of ownership of labor.

property rights over anything but a person's own labor are inherantly impossible to completely delinate (and by that i do mean ANY property other than an individual's labor).