It’s an applied field, there’s actually-existing technology that depends on it, but it’s technically challenging and a lot of people left for AI/ML because it’s easier and there’s more low-hanging fruit.

Anyway, my colleagues and I, we write monographs for each other more or less, using arXiv to announce results as a glorified mailing list—do you consider that mere “proof of work”? By my count, 250 folks is practically no one.