I don't know, but it's primarily very expensive to manufacture and hard to make expandable. You can see people in rage due to soldered RAM in this thread.

There's always tradeoffs and people propose many things. Selling those things as a product is another game entirely.

It basically looks like a games console. Its not a conceptually difficult architecture, "what if the GPU and the CPU had the same memory?". Good things indeed.

Faster and bigger SRAM cache is as complicated of a solution as adding moar boosters to your rocket. It works, but expensive. RP2040 uses ~8x more die space as its dual CPU just for the RAM.

This how the Amiga worked 40 years ago...