The reason the Demon Monarch did not just directly use its Demon Soul to take over a body was related to its identity as an Innate Calamity Beast.
The Golden-Pupilled Apes' bloodline was a double-edged sword. On one hand, they could peer into the secret of the heavens and partially conceal themselves from heavenly punishment and tribulation. On the other hand, it plagued them with the mark of calamity which attracted them to the very thing their bloodline concealed them from.