The people who contribute the most should have a proportional say in how it’s spent. This is “fairness”. Numerically there will be more people who contribute less and fewer people who contribute more under a regressive system.
All of us share the benefits of social programs and health care even if we are not directly benefiting. Society works, and people can regain health, etc. and start contributing to the overall pool of tax money again.
At no point should tax become a method to redistribute wealth - although that’s exactly what it’s become.
We can all agree that some people earn so little they don’t have to pay into the “common benefit fund” (tax) but then shouldn’t they lose the right to decide how the funds are spent?