They don't, hinting is also about removing details which you can't do in SVG.
When people provide icons in many physical sizes, the icons generally aren't mere scaling up and down, because even if you can hint so the lines themselves don't blur the details will get so small the whole icon becomes a mess. Apple even has a note on this in their HIG:
> If an icon’s content or shape is overly complex, the details can become confusing and may appear blurry at smaller sizes.
note that this is not about logical size and retina displays, it's about physical size and the viewer's eye's power of resolution.
Plus, couldn't you have multiple layers of SVG, and have the javascript respond by hiding/showing detail layers? SVG is more javascript-friendly than PNG so there's opportunity to have one SVG that gets tweaked by your js instead of multiple raster files.
Correct. But PNG works and is widely deployed today. Considering SVG fails at one of the primary use cases where it could have an edge over PNG, it's no surprise few care for it.
You can create an SVG where certain elements are hidden when rendered below a particular size using embedded JavaScript. It would be a pain in the ass because it would have to be done by hand, but you could do it. It's a problem with the tools we use to create SVGs, not the capabilities of the format.
