The problem is that while you can technically create a hypothetical infinity scenario with calculus, when it comes to the laws of physics you will eventually run into a wall.
For instance, on the math board you can indeed keep adding those 0.9's together, but lets say you are actually trying to measure a real thing like time. You will ultimately run into planck-time, where it no longer makes logical sense to make further divisions as you would violate the laws of causality. Or say you are doing a construction project, you obviously aren't going to be able to measure past the scale of splitting atoms.
This is, in layman's terms, why you can make 0.3333 infinite on the basis of abstract calculation but when slicing a pizza into 3 slices it ends up still being one pizza.