Because your ISP is probably selling your browsing history to the highest bidder, and there are a legion of data brokers out there collating every scrap of information they can get their hands on to build profiles about you.

To everyone who shrugs, and says they have nothing to hide, Would you feel comfortable wearing a T shirt in public that went into grim detail about everything you'd rather keep private, are insecure about, or might open you up to discrimination? Would you be willing to wear that to a job interview? To your bank when getting a loan?