I think it’s because the battery industry doesn’t want them.
Think about it. My tablet lasts about a day between charges. Suppose the battery was 5 times better: 5 days between charges.
BUT… a battery’s life is measured in charge/discharge cycles. Basically, how many times it can be charged. So a battery that can go 5 days between charges has a lifespan 5 times longer than that of a one day battery.
That means the battery industry would sell 1/5th the number of batteries. And they certainly don’t want to reduce their sales!
So it’s battery industry’s fault we don’t have better batteries.