For one, it's amusing to see you lean on naturalness of rights, and for another, the term "intellectual property" is a relative novelty, meant to equate the output of the intellect with goods and land, which is resembles not at all.

So, while there is a case for limited (a word which should have meaning) government-granted monopolies on products of the mind, they just don't have the same standing as rights which, if not natural to being a human, have a longer tradition than modern national governments. The word "natural" has meaning at least in terms of precedence, if not bloody in tooth and claw nature itself.

In other words, we could claim and defend a right to our lives and property and freedom to move about long before we could claim such a right to our thoughts as recorded somehow. We had to wait for government granted monopolies for the latter.

Finally, government itself holds copyright away from other rights. It's an experiment. It doesn't claim to be protecting such a right as a basis for legitimacy, but granting a limited protection. It's the difference between acquiring legitimacy and using it.