It's not like it's never occurred to them. In fact, Apple had a 3G MacBook Pro prototype a few years ago that someone snapped up on Craigslist: http://news.cnet.com/8301-27076_3-20099494-248/apple-wants-i...

A magnetically attached antenna is an interesting choice. I feel like I'd lose mine within a few days if it went to production like that.