Let's say we pass a truly universal basic income bill -- every person gets the same amount of money from the government, every month. It wouldn't take long before politicians propose giving more money to teachers and single mothers, or taking benefits away from convicted felons and sex offenders.
Over time, bit by bit, we will entitle certain groups of people to more or less money than others. That will happen for the same reason our existing welfare system is the way it is - the voters believe that some people are more deserving of help than others. UBI would not remain universal, or basic, for very long.
Given that we can't actually end up with a universal or basic income system, and that we will end up largely re-creating the same kinds of entitlements that characterize our current welfare systems, we might as well just improve and extend our current systems.