As implemented today, ECC is a feature of the memory controller. You need special ram, because instead of 8 parallel rams per bank, you need 9, and all the extra data lines to go to the controller.

Modern CPUs have integrated memory controllers, so that's why the CPU needs to support it.

Correction without reporting isn't great; anyway, you need a reporting mechanism for uncorrectable errors, or all you've done is ensure any memory errors you do experience are worse.