The ultimate reason is there isn't economic reason so. It cost's a lot to recycle anything, and most electronics would net you almost nothing valuable.

Let's look at an example. Let's say your phones main board, which will net a few hundred grams of raw materials. First thing by weight the actual board itself is probably the biggest, if you could perfectly decompose it to it's parts you would have some fiberglass, glue, a few grams of copper, and maybe a trace amount of gold. Next you would have the different components, mostly ICs but let's cover them next. These are mostly plastic with bits of copper, tin, and other more exotic metals. Most of these could be used again, if you can separate them and sort them. There would be a bunch of solder, which maybe could be reused, if you remix it with more flux. Finally, you'll have chips, these could be reused, but only as replacements for the same chip. Getting anything out of these would mostly be removing the bulk of the material which is silicon that's been contaminated with other elements to make the semiconductors. I don't think there is any process right now that could take doped Si and get you anything back. Besides the silicon you have micrograms of gold and other conductors.

Having put all that down, I think there could be an opportunity to take the bulk components off boards, test and sort them, and sell them in bulk.