Why do I have to register for every individual StackExchange community? Shouldn't I just be able to have one login to have access to the entire StackExchange family?
Not everyone wants to "be the same person" across all the stackexchange sites. If you choose to use the same identity provider, then your accounts are all connected. From my SO profile you can navigate to my profiles on cooking, science fiction, travel, gardening, the great outdoors, etc and learn actually quite a lot about me.
I may have accounts on other SE sites that you can't see from my SO profile. That would involve using a different identity provider for those sites. (And most likely a different email address, to get a different gravatar, and a different display name.) The way things are set up now makes that as easy to do as the "everything is public here's links to everything about me" way. I like that.
Usually, when I go to a site where I haven't logged in for ages, it notices some cookie in my browser, I guess that I have logged in on another site, and after a pause I get the `Welcome back, Kate Gregory, you've been logged in. Click here to Refresh" message. That feels pretty much identical to single login to me.