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.
If it wasn't for the regulations you wouldn't even have vacation or maternity leave. It's capitalism, when you're in competition, any value you can throw under the bus to become more competetive will go under the bus - either you'll do it, or one of your competitors. The answer to the question "why is my company so dysfunctional" is often "because it can't afford to not be".
In a country with statutory maternity leave the maximum maternity period is much longer than the average holiday (The UK allows up to a full year). So you have to fill the gap by hiring another developer with the same skills who is happy to work on a temporary contract which might be more difficult and expensive than finding someone seeking a permanent position especially if you require rare skills.
Industries where skills and certifications are more standarised don't seem to have this problem.
For comparison in the US: My wife and I recently took about 60 days of leave (paid) when our daughter was born, but paid leave is uncommon in many industries. Federal law requires the employer to let you take up to 12 weeks of unpaid leave after giving birth, and does not guarantee any pre-birth leave.
Why would I hire extra people just in case some of them leave? I'd wait until an employee said s/he was going to leave and then hire someone to replace them.
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.