Because it isn’t 0.9; it’s 0.999… with the ellipsis saying “repeat this to the infinite” being part of the number. And you don’t need to round it up to get 0.999… = 1, since the 9 keeps going on and on, so their difference is infinitesimally small = zero.
Another thing showing that they’re the same number is that there is no number between them. For example:
0.9 (no ellipsis) and 1 are different because 0.95 is between them
0.95 and 1 are different because 0.97 is between them
there’s no number between 0.999… (with ellipsis) and 1, so they are the same. inb4 no “last nine” because it’s infinite.