Redundancy. That's why we have 2 of a lot of things that function just fine with only 1.

If a creature gets injured, there's a chance only 1 of the 2 will be injured and that creature will still be able to have kids. So at some point maybe some ancestral creatures had only one testicle, but those were more likely to be unable to reproduce so they got kicked out of the gene pool by creatures with 2. Humans inherited this trait from whatever creatures developed this over millions of years.

It's possible nature never tried 3, but more likely other factors meant creatures with 3 didn't have advantages or had a disadvantage over creatures with 2. I can think of one that relates to a flaw in baseball: a man with 4 balls cannot walk!