Three major reasons make free education for children worldwide a governments obligation. The first one is that education is the main tool and way for progress. Thus if a country wants to ensure its future is has o to education. The main way it can do that is providing free education for them.
The second argument is that people with basic knowledge have proven that they have more tools and competences to face the problem of their lives. Therefore, I a government wants to ensure that it allows them to become independent adults capable of solving their day to day problems, or they will become dependent on a government.
The third factor is that education also provides critic and progressive ways if thinking to individuals. Therefore if a country looks to become better in every way of time. It has provide its citizens with the tools to archive it. The country will benefit from it indirectly. But it will be its best option. As countries like South Korea, Singapore, and Japan provided resources to their citizens to improve and they improved their countries.