Why do Muslims today believe the Bible has been corrupted, when Mohammad always ….?

It's not too different from many other religions, such as Mormonism and the Jehovah's Witnesses. It is from the doctrine of demons. Note that the founders claimed to receive a new revelation from God. Many founders of cults claimed that godly angels were speaking to them, and they probably legitimately believed they were good. However, the Bible tells us to beware of demons, for they come off as angels of light. I've had personal experience with demons too. They said kind words like "sweetheart," but then I figured out that they were demonic. They have been alive a lot longer than we have and are very skilled at deception. Our battle is not with flesh and blood, but with powers and principalities in high places.