Because you're making a false equivalence. It wasn't until 1989 that copyright was even implicit in the United States. That came with the ratification of the Berne Convention.
That's because the need for copyright in law is very recent. Prior to Gutenberg the act of copying books was a very laborious task. With the printing press copies could be made very easily. And with industrialization copying inventions became quite easy too. Then and only then copyright, patents and intellectual properties were conceived because for the first time in the history they became NECESSARY. However, culturally speaking many are still conceptually living in the pre-industrial world: "it's my book! I can do with it whatever I want!"
IP laws are not inalienable rights, and they have been very significantly tightened, due to heavy industry lobbying, in the United States over the last 30 years, especially with several acts passed in the 90s. We had perfectly functioning contract law before significant copyright and other IP protections. The two are not inextricably linked.
Yes, intellectual property rights ARE inalienable rights. Just because the precondition for intellectual property was not fulfilled until a few centuries ago doesn't mean that it isn't inalienable. Inalienable doesn't mean "has existed and will exist forever." When that is said let me add that I think that the IP laws of the US (like many of their other laws) are not good. I can explain what kind of IP laws I think should be practiced, if you like, but that is really irrelevant to this discussion. Before we get into the nitty gritty details of formulating good IP laws we should be able to get past the most significant bit: should or shouldn't ANY IP be protected by law at all? The reason I mention US laws here is to make sure you understand that I am not saying that just because I think that IP is an inalienable right, then all US IP laws are just.