Here are my guesses:

1 - they already do

2 - the technology isn't actually sound or reliable or is only suitable in very specific cases

3 - it's too difficult to integrate

4 - the batteries aren't rated for it, or there's FCC regulations, or EMF problems or standards that prohibit it

5 - it makes batteries explode sometimes

6 - it makes batteries too unpredictable and makes monitoring even less accurate

7 - it doesn't provide enough current when in use

8 - there's some massive industry conspiracy and they'll be coming for your children next

How'd I do?