Well bluetooth has built in security, CAN bus does not, or it's more limited.
In any case I'm hoping it will be done eventually, and I agree no reason to stop at the seats, of course. The reason is hasn't been done yet is because it's very difficult. Ideally we should have some manufacturer leak info to get us to start with somewhere. Maybe an ex-Ford Sync employee decides to defect and join the community or something like that.
Now if that ever happens, the profile of this forum will be exponentially enhanced. If done right, this is going to be like jailbraking the iPhone, people will flock to this forum to extend their car's computer abilities and a new paradigm will emerge.