People with 45 chromosomes (or to generalize to other species: individuals with mismatched chromosome counts as a result of evolutionarily recent chromosome fusion or splitting) do not generally have a high chance of being sterile. They may have a high chance of some kind of reduced fertility, though even that is not clear. There's a major detection bias here since almost no one ever gets karyotyped unless they believe they have fertility problems in the first place. This inflates estimates of how often these chromosome mismatches cause fertility problems.

The answer to the question of why any mutation that has any negative effect on fertility would spread is, as others have said, random success. In evolutionary terms, this is called genetic drift. Genetic drift is sometimes thought of as affecting neutral variation that doesn't have either positive or negative effects. However, it's been well understood from the beginning of genetic drift research that what really matters is the "strength" of genetic drift versus the "strength" of natural selection. Many things can make genetic drift stronger, like a small population or an expanding population or pops where some individuals reproduce more than others, etc. If enough of these drift exagerating factors are found in a population (as they often have to humans), then even variation with pretty strong negative effects can still spread. Beneficial mutations can also be lost in the same way.