The 4th amendment encompasses: "The right of the people to be secure in their persons, houses, papers, and effects..."
The literal text applies to your house and your person. You can make certain analogies from there, but even paper mail is a bit of a stretch (it was found to be protected because the Constitution creates the postal service). For example, one analogy might be: if I go everyday to the local news stand, the 4th amendment doesn't prevent the government from asking the news guy what magazines I look at without a warrant. So why should the government need a warrant to ask Google what things I search for on their service? I'm not saying that its the right analogy, but rather that people who say "oh the 4th amendment obviously covers information online" are starting from the result they want and working backwards.
The problem is two-fold. First, there is no general right to privacy in the Constitution. Privacy is a pretty modern concept. People in the 1700's had very little privacy even in the physical sense.[1] What the 4th amendment protects is a narrower right: the sanctity of your home and personal effects and freedom from physical government harassment. Second, we don't have a well-defined idea of privacy even today. Why should the Constitution protect information I freely give my credit card company, Google, Facebook, etc, with the knowledge they use that information to sell me crap. Why should the Constitution protect information third parties collect about me from their own observations? There might be good reasons to institute those protections, but we don't have a well-defined idea right now where the lines should be drawn.
[1] There is an interesting documentary I saw once about how hallways are a pretty modern concept that arose contemporaneously with ideas of personal space. In houses from the 1600's you'd through bedrooms to get to other parts of the house--the expectation of private space was just a lot less.
Given your statement that privacy is a relatively modern concept, I find it interesting that the EU has comparatively stronger data protection laws.
Those countries in general are way older and have older laws than the US. So if they can have strong data protection and privacy why should the US not have an equivalent?
Giving up protection in the name of commercial interest is not a good solution or compromise.
