I think it's better and more useful to ask "Why is my company so dysfunctional that I can't afford to have someone out on vacation?"
Remember that the difference between someone taking maternity leave, and someone quitting, is that people come back from maternity leave. If your company is so short-staffed that you can't afford to have someone out of office on maternity leave (or paternity leave, or even just a normal long vacation that they've saved up PTO for), then what are you going to do when those same people simply quit? The net effect is the same: you are now even more understaffed, but it's a permanent problem instead of a temporary one.
Remember that the difference between someone taking maternity leave, and someone quitting, is that people come back from maternity leave. If your company is so short-staffed that you can't afford to have someone out of office on maternity leave (or paternity leave, or even just a normal long vacation that they've saved up PTO for), then what are you going to do when those same people simply quit? The net effect is the same: you are now even more understaffed, but it's a permanent problem instead of a temporary one.