I don't understand why everybody does not block third-party cookies by default. I took a stab at my cookie list and found 300 cookies from advertisers and intel

I don't understand why everybody does not block third-party cookies by default. I took a stab at my cookie list and found 300 cookies from advertisers and intel gatherers. I deleted them selectively, but I did not want go through that again, so I blocked the third-party ones. Some have been explicitly aloud because I trust them, like google analytics. But other google cookies are prohibited, like plus.google.com. Facebook is explicitly blocked. Doubleclick is blocked. some websites will not work if certain third parties are blocked, so i have to explicitly allow them once i realize the problem.