Related: Why didn't Sauron get personally bodily involved in the events of LoTR?
Pippin feared that Sauron was come himself to Gondor:
‘Nay, not yet, Master Peregrin! He will not come save only to triumph over me when all is won. He uses others as his weapons. So do all great lords, if they are wise, Master Halfling. Or why should I sit here in my tower and think, and watch, and wait, spending even my sons? For I can still wield a brand.’
This is reflected throughout the history of Middle Earth. Going backward, in the War of the Last Alliance, Sauron only came out after the Alliance had defeated his armies in the Battle of Dagorlad and laid siege to Barad-Dur itself, a siege that lasted for seven years. In other words, he had no choice but to fight,
