I agree there's much more nuance to it but that requires far too much customization which you'll never get from a government for a country of over 328 million people. So there has to be a blanket income threshold.

Assuming $75k for a single person is a fortune is extreme, we start splitting hairs with who we think deserves more help or no. Just because someone is filed as single doesn't mean they should receive less, that person could be indirectly helping their family. Also, those with families did receive extra money for each dependent on top of the standard amount.