Fixing scoping in userland, like with styled components or BEM, is definitely doable but not really the same as having native support for scoped styles in CSS.
There's nothing wrong with those solutions and I've used those and similar plenty, but they are fragile. Those are conventions that have to be maintained and stuck with, and often for scoped style solutions you're also left with a hard dependency on a build step (BEM is an exception there). That may not be a problem at all, if you're using react you almost certainly already have a build and bundle step, but not every project is that way.
There's nothing wrong with those solutions and I've used those and similar plenty, but they are fragile. Those are conventions that have to be maintained and stuck with, and often for scoped style solutions you're also left with a hard dependency on a build step (BEM is an exception there). That may not be a problem at all, if you're using react you almost certainly already have a build and bundle step, but not every project is that way.