Boost converters from single cell alkaline batteries up to useful voltages are usually quite inefficient.
If you see something powered by two cells, it might not have a voltage converter because a lot of circuits can operate off that voltage directly.
If you use this sleeve inside a device with a built in voltage converter, you waste some energy, and your device gets confused about remaining battery life.
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?
10 - "everyone knows" overdrained batteries leak, cheaper ones worse than the expensive ones.
2 - Cost, as mentioned
3 - Some might already do it (probably down-convert the voltage to be used)
Of course, if they manage to sell their 2x lifetime batteries for twice the price it would sorta even out again. But the customer, when faced with regular batteries and 2x priced batteries, will still pick the cheaper one despite the lifetime claims.