What if the cat pictures are an app too? The computer can't require a password specifically for ransomware, just for software in general. The UI flow for cat pictures apps and ransomware will be identical.

A computer that can run arbitrary programs can necessarily run malicious ones. Useful operations are often dangerous, and a completely safe computer isn't very useful.

Some sandboxing and a little friction to reduce mistakes is usually wise, but a general-purpose computer that can't be broken through sufficiently determined misuse by its owner is broken as designed.