Because counties are administrative units, while countries are sovereign entities. One of the fundamental characteristics of a sovereign is the establishment and policing of a border. What it means to be a "country" is, among other things, some people getting together to use force to keep other people out. This was the framers' understanding of the world, a world of sovereign entities with borders and a right to police them. Within this framework of understanding, you can't say you don't reasonably expect not to be subject to a search when crossing a border. One of the basic purposes of the Constitution was to invest the federal government with primacy with respect to the national border: preserving it (collective defense), regulating the shipment of goods through it, taxing the shipment of goods through it (customs), etc.
Indeed, one of the primary ways the federal government was to be funded was through taxes on imports, which implies the existence of a customs function at the border. Do you think the framers envisioned a customs system that couldn't enforce its tariffs without getting a court order to search incoming ships? They obviously didn't envision that, because one of the first things Congress did was to set up the framework for warrant-less customs searches.
OK, I think that makes sense. I would generally object to an appeal to tradition like this, but it seems to me that the use of the word "reasonable" is basically an explicit call out to tradition, so I guess that's just how it's written.
Do you know what, if any, limits are placed on border searches? Is literally anything "reasonable" there, or are the limits just different?
The common law process is by its nature an appeal to tradition. If there is a governing principle in our society, it's "this is okay because we've always done it this way." Yes, in a way it's ridiculous because we end up arguing about what people thought about border searches in 1789, but that's the nature of our society. We don't come to a consensus to solve problems, because we never agree on anything. Instead, we litigate them and grudgingly accept the outcomes. That's why court cases feature so prominently in our culture (Brown v. Board, Roe v. Wade, etc). Non-Americans, understandably, usually find it utterly bizarre.
Re: border searches, I'm not well-versed so I'll just point you to the wikipedia page: http://en.wikipedia.org/wiki/Border_search_exception. Long story short, it still has to be reasonable, it's just that the bar is lowered because you should expect to get searched at a border.
Indeed, one of the primary ways the federal government was to be funded was through taxes on imports, which implies the existence of a customs function at the border. Do you think the framers envisioned a customs system that couldn't enforce its tariffs without getting a court order to search incoming ships? They obviously didn't envision that, because one of the first things Congress did was to set up the framework for warrant-less customs searches.