Practically, the planet is very big and injecting reflective material into the stratosphere would be a ridiculously expensive proposition, with little or no immediate economic reward for those carrying it out.

Now, when Pinatubo blew in the early 1990s, it produced about a decade-long cooling effect relative to what otherwise would have happened, but that effect faded. So, you'd have to have a plan to inject a Pinatubo-scale amount of material into the atmosphere at least once a decade, and underneath that, the fossil CO2 and CH4 emissions would steadily be increasing in the atmosphere, so if the project was ever paused, you'd have a rapid heating period which would probably be worse in immediate effect than if you'd just allow the warming to progress more steadily.

Those are the practical reasons I think it's a nonsensical proposal, along the lines of parking a sunshade in space (getting that much material out to Lagrange 1 would be so costly as to be unimaginable). It's far cheaper just to replace, say, 1-3% of fossil fuel production per year with equivalent wind/solar/storage.