Why did Lord Rama use monkeys instead of his own human army?
Apes, not monkeys. There is a difference. Now, in order to answer this question we need some context and pespective.

Rama was highly disciplined. He came from a line of ancestors who under no circumstance will fail their words once spoken. Rama was banished from his kingdom ( why ? , thts another story), and by that very logic could not use any resources or manpower of his kingdom.

He made peace with this decision and lived like a hermit with his younger brother and his wife. Only after his wife was abducted, both brothers began tracing her down with any help possible. They came across a tribe more popularly known as “Vaanar”, which in strict sense means – one who dwells in forest .

They traded help for each other where Rama will help the outcast vaanar leader to gain their kingdom and in turn the vaanars will help Rama trace her wife Sita by literally searching in all directions.

People confuse Apes with monkeys. Thats wrong. Apes are not monkeys. Apes are from the genus “HOMO-”, like we are homo sapiens. That typically means in layman’s terms tht those genus homo have more in common with us than monkeys. Since we now know that homo sapiens were NOT the only homo genus to have existed in past. There were multiple genus homo living and sharing the same land in past. Homo sapiens were one of them and the only one to survive.

Some famous genus homo are homo erectus, homo habilies and Neanderthals.”
eanderthals.

Also one story

Once upon a time , a sawayamvar was going on in paradise and narada muni want’s to marry the princess. As,narada muni was a big devotee of lord Vishnu, he went to him and asked him to make him handsome so that he will win the sawayamvar and marry the princess.

Instead of making handsome,lord Vishnu turned narada muni’s face into monkey.

Narada muni went to the sawayamvar with confidence,that he will easily win the sawayamvar.

As soon as the narada muni entered sawayamvar, everybody started laughing at his monkey face.

Narada muni couldn’t understand the reason so he went to river to wash his face. There he saw his face and got very embarrassed and went back to lord Vishnu and cursed him that he also has to suffer separation from his wife and these monkey’s will help him to get his wife back.n