A company designing a product around a battery could contract to have as many made as they wanted. They don't have to rely on batteries made for a specific phone. They could pick a design they like and stick to it, or make their own similar type. I was thinking more along the lines of off the shelf technology rather than a specific model of battery. Phone and camera batteries are pretty much all the same basic design.
There are 10 million cell phones sold for every wireless microphone sold. The market volume is not even wildly comparable.
Furthermore wireless microphones need at least an order of magnitude more power than you can get from any cell phone battery.
Cell phones transmit tiny bursts of data measured in milliseconds. Wireless mic transmitters transmit continuously for hours at a time.
Wireless mic batteries wouldn't fit inside round, hand-held microphone cases.
These would seem to be some of the reasons no wireless mic manufacturers have ever used cell phone batteries.
IMHO, it is also highly desirable to be able to use primary cells when rechargeable cells can't be recharged.
I have been on more than one tour in the 3rd world where there was no opportunity to recharge cells for days at a time.
Ideally, designers will make future products that use replaceable AA-form cells, but will tolerate lower voltages from rechargeable chemistry.