Why does my phone need a SIM?

Whould it not be easier to just authenticate with a username and password like is done everywhere else?

Imagine:
– Getting a new phone
– Borrowing a friends phone when you run out of juice
– Using a public telephone

Rumours of an actual google-phone (as in hardware designed by google) predicts a dedicated VOIP solution. Thats one of those things that make me go “why is it not allready like that?” (and then “oh – one more area google is going to dominate in a couple of years”).

You have to wonder how long carriers will be able to charge for calls seperately and by the minute. And when can i have a phone that doesn’t rely on a 20 year old SIM card?