Who says we don't know how to do it? Technology has advanced since the 1970s.
"We do not."
We do. Look into how Cyc works, and the work of Doug Lenat. Look into how Palantir works. Just because you don't know how to do it, or you haven't seen it, it doesn't mean it hasn't been done or isn't a lot easier to do now that CPUs, ram, and other technologies exist which didn't exist back then. The same reason people thought neural nets couldn't scale, until the technology got to this point where it could be scaled using GPUs. That is what changes things, the ability to have broadband, faster chips, so now it's just a matter of putting them to use.