- the whole system is waterproof, so coming up with either a waterproof rechargeable plug or a way to waterproof an accessible battery compartment is very difficult
- the transmitter is much more expensive than the sensor, which means that if you had to change the transmitter every 14 days it would be way too expensive.
The sensor is only a piece of plastic with a metal wire and no electronics. Putting a battery in it might be very difficult to isolate from environmental constraints - although not impossible, possibly the only cheap way to achieve what you are thinking.
Of course, I am not sure that the battery is a significant size issue. I am guessing it probably represents 10-15% of total transmitter volume, but I could be wrong. I wonder if anyone has opened a G5 transmitter and published the pics? A diabetic version of ifixit:-)