Two reasons: First, it's a lot of wiring which makes everything more expensive, complicated, power hungry, etc. Mobile SoCs (and Apple's chips) sidestep this is

Two reasons: First, it's a lot of wiring which makes everything more expensive, complicated, power hungry, etc. Mobile SoCs (and Apple's chips) sidestep this issue by packaging the memory with the CPU so distances are short and the issues are avoided. Second, increasing memory bandwidth wouldn't be that much of a performance win because as you point out there isn't that much parallelism and normal software would benefit much more from lower latency than higher bandwidth. Together that means a many-slots design would cost a lot but not be proportionally faster. Tradeoffs are different for server boards (many cores in the CPU) and on platforms with integrated GPUs (becoming more common) so none of this is static. I suspect in-package RAM will become more common even for platforms that still have a couple channels of slotted memory, that would create a pool of faster memory available for graphics etc while still leaving room for more if you need to have a lot of Chrome tabs open or whatever.