Almost every President until Nixon claimed the constitutional right to impound appropriations, and actually did it. And Congress often objected, but what could they do? Until Nixon did it so much, that Congress passed a law against it. And Nixon decided to sign the law, because he wanted to put the controversy over his own impoundments behind him. And from then until now, even if some Presidents questioned the constitutionality of that law, they decided to abide by it. Until finally, now in the second Trump administration, Trump has listened to conservative legal scholars arguing that the law is unconstitutional, and decided to adopt their argument and ignore it. And likely SCOTUS will decide its constitutionality as a result. But you make a decades-old debate sound like something that has a completely obvious answer. If the answer is as simple and obvious as you think it is, how did almost every President up to and including Nixon get it wrong?

From first principles, congress is given power of the purse. If the president can just ignore congress' direction and refuse to use the money they allocated, do you believe that congress still has power of the purse?

From first principles, the American Founding Fathers were largely copying the design of the British system (as they understood it), but with an elected President replacing the King. In the British system, Parliament could stop the King from spending money – but the power was about stopping the King from spending as he liked, not forcing him to spend when he didn't wish to. The English Civil War was fought over the principle that the King couldn't spend money without Parliament's authorisation; the issue of Parliament trying to force the King to spend money when he didn't want to spend it simply never came up.

Hence, look at the Appropriations Clause of the US Constitution (Article I Section 9 Clause 7): "No Money shall be drawn from the Treasury, but in Consequence of Appropriations made by Law..." That's a negative clause, a prohibition on the Executive spending money without legal authorisation; nowhere does it explicitly say that Congress can force the President to spend money when he doesn't want to spend it.

For most of the term's history, "the power of the purse" was understood as the ability of the legislature to limit government spending; the idea that it also entails the ability of the legislature to compel the executive to spend money which it doesn't wish to spend is much newer.