The average consumer knows that more "jigabits" are better and more "jigahertz" is better (see Intel NetBurst for how badly that can go wrong).
See a link elsewhere in this tread, someone posted a memory error presentation that talked about FIT, failures in time. But the average consumer doesn't know what that is.
Hence we get a race to the bottom. PC assemblers are willing to sell their mothers into slavery if it can save them $0.05 in build cost. ECC doesn't fit into that narrative.
BTW ECC is "in every computer" nowadays. As yet another poster mentioned, Intel CPUs use ECC internally to protect their caches.