The fundamental reason is that most people see the state as a protective entity. People don't give their information to someone they don't trust (at least not for free). But they fundamentally trust the government as a whole, even if they may mistrust individual parts of it [1].
This is a hard notion to combat because the government is a protective entity. It's arguably the most important function of any government: protecting its citizens. Even corrupt and dysfunctional governments can still protect their citizens from external threats.
In Western nations, one of the biggest threats to the average citizen, particularly his or her rights, is the government itself. Separation of powers is supposed to keep it in check, but at least in the U.S., the slow and steady increase in the power of the executive branch in the past century combined with technological developments has meant that it now has the potential to exercise tyrannical levels of power over virtually any person or class of person.
People don't need to understand the web or even how the technology of mass surveillance works. Citizens as a whole don't make political decisions based on detailed information anyway. Surveillance reform can only be accomplished by getting a large enough people to distrust the government itself. Skepticism won't be enough--fear of being vulnerable to terrorists (or any of the other usual fears like child pornographers and such) will trump skepticism.
In the U.S., the constitution is a document which gives limited powers to the federal government and justifies each of them. People today generally think of government power in opposite terms: it gets power by default and only has to give it up if that power later proves to be unnecessary. This is the real change that needs to be made, one of mentality not just of laws. The laws will follow if the mentality of the citizenry as a whole changes. The great deal of skepticism that younger Americans show towards government compared to their elders is encouraging, but that will only lead to change in the long run. I'm not sure what the sitution in the UK is like.
The basic problem with the main media narratives surrounding surveillance is that most (but not all) look at mass surveillance by the government and only conclude (correctly) that mass surveillance is bad and that the government better have very good reasons for doing it. Government fearmongering follows, placating the skepticism.
Mass surveillance needs to be seen for what it really is: the signature of an embryonic tyranny. Even if we presently have saint-like politicians who will only use such power for good, our government was set up so that we would not, and indeed should not, need to trust it.
[1] The surveys that regularly show high public disapproval of Congress come to mind.