If a particle kills a repairable part of the die (which can be downgraded, or cut-off) you still get from a quarter, to a half of die area dead, and a second grade chip.

But if you can make 2x dies from wafer even with the same defect rate (it will usually be less.) Even if the die will completely die from a local defect, you still get much more perfect dies in total, which is more important for your bottom line.

Standalone SRAM can be very repairable, and high yield with a custom process. Adding few spare columns, or SRAM banks should be covering for much more defects than if binning was done per entire CPU die.