I think it's one of those things that comes down to having data (from studies and such).
Studies show they improve outcomes. That's pretty clear. That means they probably also save money.
It's kind of like education, or free breakfast at schools, or drug intervention, or prison training or...???
You may have an inherent bias about the topic, and the money.
"Parents should feed their own kids"
"I never went to preschool so it's not necessary"
"You made your bed, so lie in it"
etc etc
But in these and many other cases, studies, and statistics, show positive outcomes.
If spending "X" on free breakfast means that you save "Y" because kids pay attention in school, get degrees, and have better outcomes, then it's worth it, especially when Y > X.
If spending $600 on a doula means that you have a vaginal birth instead of a C-section, then you are saving $10k or more.
If paying "A" for preschool for underprivileged kids saves you "B", and B>A, then it's worth it.
Sometimes it's important to take a step back from personal biases about whether something seems necessary, or fair, and instead look at the numbers and actual results.