The only plausible explanation I’ve heard other than “human error” is the notion that systems were “hacked” or somehow made to provide a distorted view to the crew. But the bridge wing lookouts that you refer to would make that theory untenable, because unless the lookouts were totally asleep, they should have detected the threat even if the electronic systems did not.
I read a headline last weekend that said Russia is testing GPS hacking. I'll look for it so I can post it here (which guarantees I'll never be able to find it).