That sort of direction wouldn't be meaningful anyway, since money is fungible.

It’s not like money that was previously allocated for debt service could then be moved to something else. In AZ we can direct about $4k to specific organizations and receive a 1:1 tax credit. That impacts revenues, but not how much those programs would receive otherwise.

They could obviously just issue new debt, though.

You can make tax-deductible donations to certain organizations. This results in them getting more funding than they otherwise would. Sure, redirections may not modify the budget of the US government programs directly, but it does have meaningful impact on the world.

It would be if federal budgets were determined by the allocations people indicated on their tax filings. But that would be a mess if unrestricted.