We may not be the only intelligent life in the entire universe but the only ones in our corner of it and that's why there is no signs of alien civilizations anywhere to be seen. I've also heard people much smarter than me give mathematical explanations as to why the evolution of intelligent life is way more unpropable than most people think.

Why I believe in simulation theory is just statistics; eventually we will develop technology that is able to run such simulations and we're going to create thousands of them. From that point on when new consciouss being is born it's unlikely it appears in our world instead of the thousands of simulations we're running. I just think this applies to us aswell. For us to be living in the base reality we'd either have to be extremely lucky or be the first ones to create such simulations.