Which is all well and good if you're talking about something you can easily replace.
Do realize that when someone spends $130M on a passenger jet, they expect to get a couple decades service out of it, and they aren't going to be ripping out the electronics every other year to upgrade like it's a PC - they upgrade things when they will either get more service revenue out of it (i.e. there's an RoI to be had), or if it's required on an airworthiness directive for safety.
Some of these systems were created before ECC was a thing. And more of them are using industrial components that are designed to work within a very specific range of operating conditions (temperature and power consumption being incredibly important) that even today may not be able to accept ECC RAM - for example, do you know of an off-the-shelf ARM64 CPU that can run inside of 15W and supports ECC? Neither does anyone else. Yet there are companies that are making avionics hardware that is not part of critical safety systems that run on ARM64 because of the power / heat dissipation problems.
It's a very difficult problem space without getting inordinately expensive due to having to make custom hardware at every single turn.