If a child lives with a parent on SSI the parent receives an extra $250 month to take care of their child. This is for the first child. Any additional children add $150 month each.
In my state if you are a licensed foster parent you receive $468 a month for a child placed with you. (They will place a child with you and then spend months finishing your licensing paperwork. In which case you get nothing until the paperwork is done. No back compensation either)
Evidently the government thinks it costs more for a non family member to raise a child? Not to mention every foster family I know personally has a family income way above what SSI pays.