Manufacturing processes for RAM and processor cores are not one and the same and use different tradeoffs. It's the main problem in-memory processing is running into, which is akin to what you're describing here. The die space required for the same level of performance makes the idea of on-die RAM intractible. The best they're able to do with RAM optimised manufacturing processes are tiny RISC-V cores that lack most of the feature sets. What apple does with moving the RAM chips on to the package really is the only viable option.