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.
Industries where skills and certifications are more standarised don't seem to have this problem.