Assuming you're referring to Apple Silicon's memory bandwidth, that is not necessarily because the memory is on-die. The bandwidth comes from having more channels to access memory. This gives the SoC a wider bus to increase throughput vs. your typical x86 system with two channels. For whatever reasons Intel/AMD decided that two channels is all the typical consumer chips can support now so it's on them.

On laptops right? Weve seen more channels for years elsewhere

Ah I see, thanks for breaking it down.