Because history, that's why. FISA -- in its original form, not the "War on Terror"-era "FISA Amendments Act" -- was a limitation on government surveillance power which criminalized all sorts of surveillance except in very narrow circumstances, that was adopted not because of theoretical concerns but because of widespread, notorious, abuse of the national security intelligence-gathering capabilities of the US government for political purposes by the executive branch of the United States government, particularly during the Nixon Administration.
These limitations were weakened through the FISA Amendments Act on the excuse that the limitations were "hamstringing" our intelligence agencies and jeopardizing our national security (after it became publicly clear that, in fact, the limitations were being widely ignored, and the current administration wanted legal cover for actions that it was already undertaking despite the law), and, hey, look what we see -- massive, unfocussed, broad-spectrum surveillance of exactly the type we were assured wouldn't happen if we loosened the reins a bit so that the executive branch had the tools it supposedly needed to focus on direct threats from terrorists without overly burdensome constraints.
The beauty of the modern world is that the government does not have to violate the sanctity of your home or person to get tons of information about you. You freely give it away: to Google, AT&T, Facebook, etc. Nothing about the 4th amendment prohibits them from going and asking these entities for information they have about you or that you have shared with them.
This is much like if the NSA had an agent whose job it was to go through bank strongboxes - all of them - examining the contents and looking for things that are illegal. It is the bank's security that is violated, not your own, but it's still your "papers and effects" that are not secure against unreasonable searches.
I completely understand the judicial argument that has been made amounting to "Welp, it's on Google's servers, you don't have any right to control it anymore", but as far as the spirit of the Constitution goes, that's complete bunk, and I think that any reasonable, average person would agree with that sentiment. My data, stored on Google's servers, is mine, and is entrusted to them for safekeeping.
The government has managed to finagle its way into an interpretation of the Fourth that utterly defangs it. The purpose is "Hey, you can't go through peoples' stuff without judicial oversight", and that is clearly being violated as a matter of routine.
Second, you're wrong about the "spirit" of the 4th amendment being about the government not searching through your stuff. It's about not violating the sanctity of your home or person to do so. That's a very key distinction. Even in 1802, the government could, pursuant to a court order or even just consent, search through "your stuff" that you had left for safekeeping at a friend's house.
I think where this is sticking for me is the whole "consent" issue. I have a very difficult time believing that all the accused companies have voluntarily consented - without coercion - to NSA taps. If that's the case, then welp, all bets are off. I don't think that's the case, though - if it has happened, it is almost certainly under some sort of coercion, at which point it's very difficult to argue that this data is being given with free consent, and we're back to it effectively being seized via "consent" given under duress (which any court would strike down as not being consent at all!)
Regarding the language itself, the 4th says you have the right to be secure in your papers (personal data!) and effects (personal property) against search and seizure, not just the sanctity of your home and person. I would like to know how that is reconciled with modern legal interpretations - that seems like a big gaping hole to be filled.
Maybe I'm just naive and want to believe that things are better than they are. Maybe Google et al have just rolled over and happily handed over data. I really have a hard time believing that's the case. In either case, though, we have allegations of our government conducting wide-scale spying on its own citizens, by means of the seizure of those citizens' supposedly-private communications. Given a country that is supposed to uphold the values of liberty and individual freedom, that is extraordinarily distressing.
Duress is not coextensive with coercion. Consent can be valid even if it involves leverage (e.g. no government contracts for you unless you consent).
> Regarding the language itself, the 4th says you have the right to be secure in your papers (personal data!) and effects (personal property) against search and seizure, not just the sanctity of your home and person.
4th amendment interpretation does make the logical analog between "papers" and "files" or "data." But information that you've given over to others (e.g. files stored in your Drop Box) or information that was not even generated by you (e.g. server logs, call data records) is not your "personal data" whether it's ink on papyrus or bits.
What's tripping you up is the illusion of "network transparency." You might think of your gdrive as the same as your hard drive, but of course it's not. The difference between the two is nothing less than keeping information about your finances in your desk (where the 4th amendment protects it), and keeping that information in your accountant's office (where the 4th amendment does not protect it).
There's actually considerable current debate (and it will no doubt accelerate with the recent revelations) over the extent to which the "persons, houses, papers, and effects" part of the Fourth Amendment extends beyond ones physical body and tangible property, but its eminently clear that it does because the 4th Amendment is the basis of the general requirement of warrants for wiretaps, even when the wiretaps do not involve a "physical penetration of a constitutionally protected area". [1]
[1] Katz v. United States, 389 U.S. 347 (1967)
Burning the whole village to save it, cure worse than the disease, cutting noses to spite faces, and all that.