Why do Persians look permanently unimpressed even when they are purring like tiny engines?

It's the face structure. Persians have that brachycephalic head shape-short muzzle, big round eyes set wide apart, and a low nose bridge. That combination creates a natural frown or "grumpy" expression, even when the cat is completely relaxed. The flat face doesn't have the same muscle mobility as a typical cat's, so you don't see the subtle eye and whisker shifts that signal contentment in other breeds.

The purring is real. I've had Persians that would vibrate like a phone on a table while looking like they were judging my entire life. The disconnect is purely anatomical-their default resting face just happens to look unimpressed. If you watch their ears and tail, you'll see the real mood. Ears forward and slightly to the sides, tail still or gently twitching? That's a happy cat, even if the face says otherwise.