That's not true. Several people have a clue about how they work. There are interesting and surprising behaviors, as they scale up. It's hard to hold the whole thing in your head (roughly impossible) the same way no single human can fit a whole modern CPU design in their head. But the math that makes it work is relatively simple and comprehensible to many people. There's just too much complexity for a human to deal with...but any individual part, and small LLM implementations are entirely do-able by a single human with no agent or team assistance.