According to the theory of constructivism the "one" reality doesn't exist. If there was only one human being on earth there would only be their subjective reality. But as we are mostly social creatures reality is based on societal consensus. Why is green green? Why is a tree a tree. Why do we define rain as bad weather? Not to talk about moral standards and values. And even the question: Am I real? I highly recommend The Tree of Knowledge: The Biological Roots of Human Understanding by Maturana/Valera. I found it very interesting.

Truth is reality. People believe and tell lies because they don't want to accept what is real. Truth comes with responsibilities and consequences. People would like to avoid responsibility and do as they please without facing the consequences. You can believe as hard as you please that you can fly, but if you jump off a cliff without a parachute you are still going to end up dead or badly injured, because you're subject to the law of gravity. That's reality. Denial of reality is what we call by definition insanity.

We live in a simulation so.....

Uhhh, nope to the previous response. Reality is not a confluence of the internal and external given that specific question, unless of course, you are a raging narcissist….

This is a false dichotomy. The internal and external world are one.