The only thing that we can know 100% for certain is "I think therefore I am" or probably better worded "I am experiencing something therefore something must exist that can experience things". There are a lot of definitions of consciousness and sentience but I think the best one is talking about the "I experience" or the "I think" in those sentences.

All of our beliefs and knowledge, including belief that the world is real must be built on top "I think therefore I am". It seems weird to throw away the one thing we know is 100% true, because of something that is derived (science, real world observations) from that true thing.