Because that's what they take from you if you underpay (by accident or not).
Living in the US is like: "oh you were born and live in the US? We're forcing you to give us a huge chunk of your earnings for your entire working life, make it close to impossible to determine the exact amounts and then fine you 5% interest if you happen to underpay, but we'll never give you interest if you overpay.".
Edit: Well, as mentioned the tax system is super complex here and I obviously don't know the rules for every case. It seems in SOME cases you do get interest when the IRS owes you money but personally I've never seen this, where as I've always had to pay x% interest if I ever owed anything.