It is the moral issue.
You seem to think that any policy can be analysed in terms of (economic) benefit to the government. You ask "why is it in the government's interest". My case is that governments often don't act in their own economic interest.
Governments are not just machines generating profit. They are composed of people who started doing their job because they thought that they could make their country a better place for the people living in it (better and not just richer). Politicians are moral creatures. The Law of the Land is a written expression of the moral choices made by politicians over the years.
In all countries, the political leaders can act to prevent something just because it is wrong. As an example, consider the UK Hunting Act (2004) which bans the hunting of wild mammals (eg foxes) with dogs. There is very little economic case for banning fox hunting. The foxes can't vote. The reason for the ban was moral. A majority in Parliament believed that hunting with dogs was wrong.
Many countries have anti discrimination laws. In India, discrimination by caste is illegal. The reason is not the economic interest of the government, it is moral.
The law in China is weaker (as might be expected from a totalitarian government), but Chinese employment law disallows discrimination by gender. The reason for this law is again because the Chinese government want to disallow actions that they see as wrong. There does not need to be an economic argument for such a law, (and the Chinese government doesn't need to worry about votes.) The fact that something is wrong is sufficient reason to legislate
Now creating a gender imbalance in society may have some negative impact, which would add to the other social issues that India and China face. But this is not the reason that it is "a very large issue". Instead, the political leaders see sex-selective abortion as a moral failure and so create policies to act against it.