Because a) it's the default, b) nobody remembers (or knows) to change it, and c) only folks like us even care.

In case anyone is interested, in Chrome (Mac at least), it's under Settings > Show advanced settings > Content Settings (under Privacy) and then the checkbox "Block third-party cookies and site data". http://www.evernote.com/shard/s8/sh/eb3a23ef-8902-4ae1-87d8-...

So I guess this is one reason why people leave that enabled: not quite obvious.

What sites, out of curiosity? Obviously all the like-like (likesque) buttons would no longer function, but I don't care about those. I was wondering what valid use case there is for 3rd party cookies that actually benefits the user and not the service provider.