Because your Form 1040 is not finalized.  Look at the list of forms that are not yet finalized in the Review section of the program.

Have you received all your W-2 and 1099's for 2018?

Why do you want to file early anyway?  The IRS has yet to publish a date when they will accept any type of tax return for tax year 2018.  With the tax code changes, tax forms change and the partial government shut down the IRS is not expected to start receiving tax returns until late January or early February at the soonest.