Why do astronomers think comets originate in the Oort Cloud?
The theory of the Oort cloud was developed to explain discrepancy of the assumed age of the universe with the observed age of the comets.
Based on the half life of uranium the age of the earth and the solar system is estimated to be 4.5 to 5 billion years old. The theories of uniform geology and Darwinian evolution demand an old earth and solar system. The consensus of the scientific community is that the solar system then is 4.5 billion years approximately.
Based on our observations of comets new ones appear that had not been previously observed. Old ones like Haley's comet burn out on a regular basis. The age of the solar system based on the duration of comets orbiting the sun would be in the thousands or tens of thousands of years old. This is assuming that the comets were formed at the same time as the solar system.
The discrepancy between the estimated age of the solar system based on other theories and radioactive half lives and the observations of comets creates a problem. The answer to this problem was the hypothesis of the Oort belt where the comets were formed at the beginning of the solar system. The theory is that passing object in space periodically dislodge some of the comets from their orbits in the oort belt sending them into orbits closer to the sun. Hence the comets observed are as old as the solar system but in new orbits.
The theory of the Oort belt resolves the problem of the age of the solar system with the age of observable comets. The Oort belt has never been observed. There is no direct evidence of the existence of the Oort belt. Scientists think that the Oort belt exists because it explains the origin of comets in a way constant with the theories of the origin of the solar system