I doubt there is any concerted effort to keep the field small. That would be like saying tech companies don't want people learning how to code so that they can maintain an advantage.

If anything, governments and companies are encouraging people to study cryptography so that they are able to hire more experts in the future.

Now, once you get gatekeeper organizations and special licensing organizations like contractors licensing or beauticians licensing, those are examples of groups trying to keep the pool of experts small.