The Presidents don't have the final say in how much money each agency gets and how many Congressmen get a shiny new NASA facility or contract in their district. The slice of the budget that the executive branch has the authority to allocate is miniscule compared to debt payments, entitlements, and military spending.

NASA's problem isn't really the money, it's the ridiculous bureaucracy and systemic inneficies that inevitably occur when you are forced by opportunists to fragment your supply chain and distribute your engineering infrastructure across the entire country. The constant change in executive policy is a consequence of NASA's inability to achieve its potential in the face of a functionaly hostile legislature.