Concordo com os pontos apresentados por Victim of Circumstance, Jacob C. e Ian Thompson em suas respostas. Eu gostaria de acrescentar os seguintes pontos.
Ele não tinha tempo nem recursos para experimentar todas as estratégias que podemos imaginar
Poderoso como era, Sauron não tem uma quantidade infinita de tempo ou recursos. A busca por dragões teria tirado recursos de outros esforços, tais como recrutar os Haradrim e os Easterlings, que ele poderia ter pensado mais em dar frutos.
Ele não achou que precisava de ajuda adicional para vencer as batalhas
Além disso, de acordo com sua sabedoria, Sauron sabia que as forças que ele tinha eram suficientes para garantir sua vitória. Ao usar o palantír, Denethor está convencido da vitória certa de Sauron.
‘My lords,’ said Gandalf, ‘listen to the words of the Steward of Gondor before he died: You may triumph on the fields of the Pelennor for a day, but against the Power that has now arisen there is no victory.
The Lord of the Rings Book Five, Chapter 9: The Last Debate
Page 878 (Single volume 50th Anniversary Edition)
Sauron percebe que o Anel é tudo o que é importante
Gandalf explica isso para Aragorn, Legolas e Gimli:
The Enemy, of course, has long known that the Ring is abroad, and that it is borne by a hobbit. He knows now the number of our Company that set out from Rivendell, and the kind of each of us. But he does not yet perceive our purpose clearly. He supposes that we were all going to Minas Tirith; for that is what he would himself have done in our place. And according to his wisdom it would have been a heavy stroke against his power. Indeed he is in great fear, not knowing what mighty one may suddenly appear, wielding the Ring, and assailing him with war, seeking to cast him down and take his place. That we should wish to cast him down and have no one in his place is not a thought that occurs to his mind. That we should try to destroy the Ring itself has not yet entered into his darkest dream. In which no doubt you will see our good fortune and our hope.
The Lord of the Rings Book Three, Chapter 5: The White Rider
Pages 496-7 (Single volume 50th Anniversary Edition)