When you work on proprietary code with a lot of trade secrets contained in it, on a codebase that did cost millions of dollars of man-hours to build and that holds the company's IP, you tend to be very careful where you're sending that to.
Source: I work on such code. We don't allow devs to use (cloud-based) LLMs.
Obviously most developers know better, but you know who doesn't care what you think? The owner(s) of the business, and various other stakeholders. Try explaining this concept to them, and see if they get it :)
The people who work in codebases that are tightly guarded aren't guarded because the developers say so, of course.