Because pandering to people like that only reinforces their behaviour and encourages them to behave in that way.
But they continue to behave that way, and the children continue to suffer from a poor diet.
Why not consider alternative ways of getting parents involved in better choices for their children, when it is clear that a school's approach is only generating stubborn opposition?
The definition of insanity is repeating the same fruitless action and expecting a different result.
You can't start out with the basic attitude that parents are The Enemy and hope to make a dent in the lifestyle choices they make for their children. People can sense that attitude, and it only reinforces the oppositional dynamic.
Nobody likes to be patronised either.
You have to meet parents where they are, not where you would like them to be.
www.edutopia.org/blog/high-poverty-schools-engage-families-community-william-parrett-kathleen-budge
This is from the US and concerns issues related to academic progress, but there are some points that could be translated to British practice, especially since some of the more sustained resistance to school lunch policy seems to come from places where academic performance is an issue too.
"Without a trusting environment in our classroom and with the families of my kids, it's all uphill. We never make the progress we could. . . we never can 'click.' Trust is what makes it all happen for us." The development of trusting relationships lies at the heart of successfully engaging parents, families, and the community.
There seems to be a lot of disconnect between schools and the communities they serve, and I suspect the lunchbox run-ins are only a symptom of deeper issues. The site I linked focuses on creating a more holistic, intertwined school community, with the wider community involved too.
Schools really cannot continue to ignore the way ongoing generational poverty affects engagement with school and with the aims of school. (I am not saying everyone who is poor makes X or Y counter productive choice, but suggesting that for many, poverty and associated factors complicates everything).