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.
Dancing Treefrog 
Scientists at the University of California Riverside have been focused on nano silicon for a while, but it’s been degrading too quickly and is tough to produce in large quantities. By using sand it can be purified, powdered then ground with salt and magnesium before being heated to remove oxygen resulting in pure silicon. This is porous and three-dimensional which helps in performance and, potentially, the life-span of the batteries. We originally picked up on this research in 2014 and now it’s coming to fruition.