Did Gautama Buddha believe in the existence of soul in human body? If not, why?
What is a "soul"?
In the Christian New Testament, the "soul" is most often described as the human conscience that becomes defiled or purified, similar to the mind-heart ("citta") taught by the Lord Buddha.
In later-day-Buddhism, the idea of "relinking consciousness" was manufactured; which is similar to the meta-physical idea of the Hindu soul that leaves one physical body at the end of life and enters into a new physical body. This idea is alien to the Lord Buddha.
The Lord Buddha himself was not concerned with meta-physics but with ending suffering. Therefore, the Lord Buddha was not concerned with a meta-physical soul but with the egoistic covetousness & possessiveness that creates suffering.
While the Buddha's teaching of not-self (anatta) is a natural reality; the important reason it was taught is it ends suffering. If anatta did not end suffering, the Buddha would not have taught it.