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.

You mentioned Strix Halo, which also has off-die memory. Strix Halo does have a real advantage from its wider memory bus (four channels for 256 bit instead of 128 bit), but Strix Point is equivalent-ish to Intel's platforms like Panther Lake or Arrow Lake in terms of memory setup.

In fact, Intel also had Lunar Lake, which had on-package memory. However, it was still limited to 128-bit dual-channel, so there weren't really many performance benefits; it did however help with power efficiency.

Macs or other competing systems don't have on-die memory.

(Except for the caches, which everybody has)

Nonsense, Apple has on package memory and the primary reason for that is overall packaging and layout not performance

I appreciate everyone's corrections here, my apologies. I clearly misunderstood the situation.