> why is it so hard to get a cryptographically secure random number? Because computers are pure unadulterated determinism

Then you've missed the part of software.

Software isn't computer science, it's not always about code. It's about solving problems in a way we can control and manufacture.

If we needed random numbers, we could easily use a hardware that uses some physics property, or we could pull in an observation from an api like the weather. We don't do these things because pseudo-random is good enough, and other solutions have drawbacks (like requiring an internet for api calls). But that doesn't mean software can't solve these problems.