Because the U in UBI means "universal". Saying this as someone that isn't strongly in favor of UBI.

Look why don't you look up the concept historically and how it is used now? Its surface level stuff this.

edit: To give you an answer Welfare is given to those that need it. A universal basic income is not given to those that need it, but by definition given to everyone as income.

We can redefine the "universal" part to mean "universal to those who need it ('need' by some arbitrary regularly changing criterion)". :-)

Then by definition it's not universal and it's not UBI anymore ...

Politicians only care about definitions if they serve their agenda.

... people care about definitions. We all care about definitions. What are you even talking about? We're not politicians and we're arguing definitions. Dude, go out and touch grass.