Well that comes from a lack of knowledge, imo, you just hear the world bible and it is already bs for you.
Very simpel said: Muhammad saws was the final prophet, he got revelation from God, not in one instance but over a course of ca 20 years. How do i know this? That is history, there are reports, sources, people, witnesses. And all say, Muhammad was indeed a prophet of God, if you have a problem with this, bring your proof he was not a prophet and what he preached was falsehood. However no one today has managed to do this yet. The Quran revelations were always revealed during important events or moments, not just randomly.