Which is why other Western countries offer the same benefits to part-time employees on a pro-rata basis.
Work 5 days a week? We give you 20 days annual leave.
Only work 3 days? We give you 12 days annual leave.
Both circumstances result in an employee getting 4 weeks of paid leave a year.
We also let people accrue leave across multiple years, so they can take a 12 week vacation after 3 years if they wanted to.