Again, not everyone has a cell phone. For a lot of people such as myself, they're not a viable option, whether finanicially, or just because there's no need for one. I'm never so far from a phone that I think "it would be great if I could call someone right now without walking to the nearest phone.
Sure, there's other ways of doing the authentication, but..it really shouldn't ever be something mandatory. It's a fucking email account. If someone actually wants to have to jump through hoops for extra security, then they can have a normal account and use another one just for hoopjumping. I know that's what I'll do, and it seems like a really good idea.
Personally, I find this to be a case of "most people are so stupid about security and passwords that we're just going to try to please everyone".