Fabricating them has proven to be very challenging. A bit like with solar cells with high efficiency. Labs create them, but fabricating it in useful volumes is

Fabricating them has proven to be very challenging. A bit like with solar cells with high efficiency. Labs create them, but fabricating it in useful volumes is also hugely challenging. Or, if you like, GUIs, which were demonstrated in 1967 but didn't come into great use until about 20 years later.