School lunch programs across the world get setup and perpetuated for a variety of reasons. They also have a variety of funding models.
For the US specifically, major federal programs began during the Great Depression as a two for one combo. It solved the direct problem of... people being poor and their kids not having food/lunch, and it also provided a reasonable supply sink for the government to buy out supply from farmers to help keep things going.
Anyhow, since then for a variety of reasons, subsidized/free lunches have stuck around. Primarily because the underlying problem (food insecurity) has not been adequately solved. School lunches also tends to be amongst the more politically palatable/defensible forms of welfare in the United States, since its very structure and beneficiaries make it harder to criticize.
So while expansion of SNAP or other programs that might help tackle general food insecurity might run into headwinds, most of those arguments tend to falter when it comes to feeding children directly at school. For example, it's hard to argue that getting free lunches at school would encourage "abuse and malaise" amongst students. Similarly, since the composition of lunches tend to be under control of the supplying organization, there's reduced concern of people spending their assistance on "luxuries".