I'll take a stab at it, since I'm one of those privacy advocates (and also prone to making sweeping statements like this).
Let's say Alice and Bob are doing life and emailing each other about normal life stuff. Charlie runs their email server.
Charlie also runs an advertising business to fund his email server. He somehow reads (not necessarily manually, but the details don't matter) the emails coming through his server to learn what people are more likely to be interested in buying. Everyone benefits, right? Alice and Bob get free email, the advertisers get well-targeted ads, and Charlie gets paid by the advertisers.
Well, along comes the Police. They know that Charlie is able to access contents of emails going through his server, because it's how he funds his email server. The police would need a warrant to search Alice and Bob's communication for something that might incriminate them in an investigation, but Charlie doesn't need a warrant. The police strike a deal with Charlie of mutual benefit. Information for another revenue stream. But still, the police are upholders of justice and only use this "email tap" for good.
Time goes on and our glorious democracy erodes into an autocratic state (ask Germany - it happens!). Suddenly our justice-loving Police have become the Gestapo, but money talks and it's in Charlie's interest to stay on the Gestapo's good side, so the email tap remains in place and we have Alice and Bob, good people that they are, collaborating on how to resist the autocratic state, which gets funneled straight to the Gestapo. Bad guys win.
Essentially it boils down to this: the means for the public to resist tyranny is a necessary prerequisite for freedom. Conversely, the more power (and information is power, especially personal information) is centralized, the more impactful a potential hostile takeover becomes, and the easier to orchestrate (much easier to infiltrate/control one source of information than thousands).