It's actually really fascinating that there isn't a scientific theory of deep learning, especially as it's a product of human engineering as opposed to e.g. biology or particle physics.

There are very good reasons why it took this long, but can be summed up as: everyone was looking in the wrong place. Deep learning breaks a hundred years of statistical intuition, and you don't move a ship that large quickly.

There is, but it is fractured. I would equate this effort as more of a standardization of terms and language.