it is said that Lord Brahma directed "SURAS" and they began to parent sons in the semblance of monkeys (Ramayana 1.17.8). The Vanaras took birth in bears and monkeys attaining the shape and valor of the gods and goddesses who created them. why Brahma did so? He can as well could ask them to parent HUMAN.
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In the past once, Naarada Muni with self ego, spoke to Lord Srimannarayana , that he was unmoved to MANMADHA. Lord to curb the sage’s ego He has used His MAAYA. Sage Naarada came under the clutches of MAAYA, found her a very beautiful girl. Wishing to marry her , he requested Sree Hari to have the face of Hari., .Hari also means monkey so Vishnu gave Him that face so Narada got angry and cursed Him to seek the friendship of monkeys while searching for his lost wife in next incarnation.
It was due to that curse by Narada Lord Vishnu who took avatar as RAAMA had to seek the friendship of monkeys (vaanaras) to search for Sita, and to kill Raavana .
Brahma had created JAMBAVANTA (bhallukam) to help Raama in war with Raavana.
raama charita maanas by Tulasi
The main reason behind not taking help of humans is Sri Ram was exiled and for 14 years he was suppopsed to stay in the forest. At various instances he himself told that he cannot enter into a village (while talking to Nishadraj) or fort (while talking to Sugriva) or town (while talking to Vibhishana). So being in forest one cannot assemble an army of human beings.