You touch on the primary flaw of "spend the money here on Earth" argument.

Money spent on space programs IS spent here on Earth. The money paid to engineers and fabricators and for raw materials goes to pay for house payments, durable goods, food and taxes of the individuals working on these projects. And those payments create jobs delivering these services, which benefits an ever increasing number of people.

It's not as if dollar bills were litterally burned to provide rocket thrust.