Because the penalty is calculated on Form 2210.  You can't pay the penalty without including the form.  You have the option to check the box that says ''let the IRS calculate the penalty''.  If you mark that box, Form 2210 will not be included in your return.  They will either waive it or send you a bill later.  

If you intend on paying the penalty or applying for a waiver, Form 2210 must be included in your return.