This isn't at all about discrete GPUs, where did you get that from? This is about APU/iGPUs. In general memory channel are expensive so you won't see a new product with 8 channels running 3000Mhz memory speed or something like that.
Also Apple laptops do have large memory busses. The Apple M1 Max chip for example has a 512bit memory bus.
I assumed when you said "Only enthusiast laptops go with a large number of memory channels" you intended to refer to something beyond just the MacBook Pro, because "enthusiast laptop" is usually used to describe a category that definitely includes a lot of laptops that aren't MacBook Pros, and only arguably includes some MacBook Pros. And if you did mean to refer specifically and only to MacBook Pros, there were plenty of more straightforward ways for you to do so.
It's quite simple really. Macbook Pro M1 Max is in the set of enthusiast laptops. You said there are none. I gave that as a counterexample. I wasn't specifically thinking about it in my original comment. In fact I wasn't specifically thinking about even a bus width that I consider "large". It was a general comment that memory channels are not cheap to have in a piece of silicon and that consumers generally don't care about it as a metric of performance.
Also Apple laptops do have large memory busses. The Apple M1 Max chip for example has a 512bit memory bus.