Why Don't Flight Recorders Float?

In the wake of the horrible accident which brought down Air France flight 447, it would seem that the recovery ships, divers and submarines are going to have a tough task in finding the flight recorders from the Airbus A330. Some say they will never find the flight recorders since they could have sunk in the ocean to depths of between 8,000 and 15,000 feet.

The actual Airbus A330 (tail number F-GZCP) used in Air France flight 447, sitting on the tarmac at an American airport in 2006. Click for larger version

This immediately begs the question... why the heck don't flight recorders float? I cannot understand this, especially when so many flights cross oceans and seas at some point in their journey.

Surely they could be encased in styrofoam, or some other porous material which would float? Or if the space taken up the recorders is a concern, why not wrap them in some kind of inflatable which only fills with air on impact, like an airbag?

In addition, they only continue to send signals out for thirty days which makes it a race against time to find the recorders after an accident. Surely with modern battery technology, this could be improved?

And finally, if signals can be sent out via the ACARS system to inform air traffic controllers of the status of an aircraft, why can't they expand its capability and utilise it to send out verbose messages which contain all or most of the telemetry from the flight data recorders?

This all makes absolutely no sense to me. I must find the answer...