ECC memory doesn't do anything to help when the bits that get flipped are in the CPU. Or anywhere else that isn't a RAM chip.
Except that the RAM has hundreds or thousands of times as many bits as a CPU, and Flash may have millions of times as many, and dynamic ram has smaller feature size, and is more susceptible to SEUs. So correcting RAM and Flash helps because that is where 99.9% of the problem is.
Even within the CPU, most transistors are used to implement cache, and cache can also be scrubbed (although not with just software).