When apple comes up with flexible PCB, I will pitch to them my own invention. It is truly innovative.
I take a block of copper and extrude it into a very thin cylindrical shape then dip it in some molten rubber
The amazing thing about this is that the copper not only conducts electricity but it can also be used to send data and is flexible and the rubber prevents it from being shorted.
If you make the board flexible, you may end up with a layout that only supports 1 or 2 devices and if too much flexing causes wear and tear, you may end up having to replace the entire motherboard.
But if the same thing happens to a object made out of copper and rubber, you will have a significantly cheaper fix.
PS I have made flexible circuit boards, it is called the poor mans motherboard, you take small wires and solder them to all of the needed components, then you cover everything in clear tape, you now have a flexible board