Some of that is due to the quarterly reporting requirements of many large (often public) companies. Once you go over a quarterly-- and especially a yearly-- boundary, that lack of spend gets "locked in" as profit. At that point it's subject to corporate taxes, and potentially even a portion returned to the stockholders or however else the corporate governance has agreed to disposition profits.

Amazon famously operated at a zero profit for a decade, re-investing all profits and owing no taxes on no income. If some manager had claimed to need a few million then didn't spend it (but prevented it from being invested elsewhere in the company), they'd probably be fired, not celebrated for "saving" money.

If you're tracking your spend throughout the year, and not spending the expected amount in one area, there are also potentially other ways to spend that money to get a better return than it sitting in someone's "unspent" account as cash. And if a manager is going to argue they still need it, they'd darn well better need it.