At the time it was believed (by Chomsky, etc) that natural language could be described in formal terms and parsed by fixed rules. But after several decades of failed parsers (and the success of statistical methods like LLMs), it is clear that formal and natural are fundamentally different types of languages.
Ah, that makes sense. I've always wondered why SHRDLU seemed so powerful—and yet nothing ever followed up from it, you couldn't run it, there was no "we took SHRDLU and improved it". Just the same couple bits of example dialog. I've wondered if maybe it was fake? But I guess it makes more sense that it was just a very brittle demo. Like the software equivalent of a genetic sport.