Because, technically, you’re supposed to pay taxes quarterly.
The IRS is nice enough to allow you to have taxes taken from your paychecks and sent to them ahead of time, and you only need to reconcile once a year. But, if it turns out that your deductions weren’t enough to cover a certain % of your total tax burden, the IRS will levy a penalty for not paying your taxes appropriately in each quarter.
If you don’t have deductions made, you are supposed to pay each quarter, although, for most people, this can be an estimated amount which you then reconcile on April 15 like everyone else.
If you were allowed to leave $5000 in a tax haven all year, then get it all back, minus taxes, the IRS would have been shorted money you owed earlier in the year.
That’s not really a great explanation, but it beats my HR office’s stance of “because it is a benefit to you, so you must only take advantage of it in the way it was intended”