It requires more expensive, compatible DRAM right? Knowing that, it shouldn't really be super surprising. Enabling it on die is just one piece of the equation.
The problem is that if you say "fast, cheap, reliable, pick two" people will pick "fast" and "cheap". Even people who might ordinarily worry about whether their workstation is silently corrupting their data.
i3 are dual cores that support ECC, aimed at NAS and the like (hence mini ITX boards with ECC SODIMMs). But you still need an explicit workstation chipset/mobo to support it.
i5 and i7 have no ECC because there are equivalent Xeons. At this level "Xeon" is just branding implying features like ECC, and it doesn't automatically mean expensive - a single socket non-E (LGA1150) Xeon build costs only a little more (~20%) than consumer cpu/mobo/ram (although there's better sales on the consumer stuff).
It used to be the case that all AMD CPUs supported ECC back in the AM2/AM3 era, apparently this may no longer be true though. Not all motherboards bothered to route the extra traces required for it and include BIOS support though.
All AMD FX and Opteron CPUs support ECC. The APUs do not. For the CPUs that support ECC, it is still up to the motherboard manufacturer to support it on their end as well.
