But let's assume you really want more compute Apple has to first design a standalone GPU, then it has to do a PCI or CXL core or some custom interconnect, then it has to redesign it's OS for non-uniform memory doing all that fun stuff hiding the memory communication, then it has to add the primitives to it's languages for you to manage memory (because to get performance you have to actively design for it) and then you need to get all your performance critical applications to re-write their stacks with this in mind.
It's a massive lift! And for what? Only Desktop can take advantage of it, and in Apple's product line only the Mac Pro. And that's assume you execute well and the device ends up more capable than just using the SoC. And all of this we're talking about on top of the massive lift Apple literally just finished moving all their products onto Apple silicon. It's just a crazy ask for a v1 of a product.