Ever wondered why Photoshop doesn’t have real-time line-smoothing for its brushes? It’s probably a market-driven thing, rather than about the technology. My guess is it’s something to do with not wanting to cross into Adobe Illustrator territory.
Anyway, there is a way to get excellent line-smoothing in Photoshop. You just install the Lazy Nezumi Pro plugin. The software’s landing page makes it look rather complicated, but that’s mostly just the settings options. It’s actually quite simple, and you’ll get good results with default presets. It’s very light on your PC system resources, too. It’s only for Windows.
Also works with Corel Painter, Sketchbook Pro, Manga Studio and others, though they all have line-smoothing in one form or another.