The short answer is this:

If man on Earth ever progresses to the point where they can create Earth (and everything on it) simulations so good it is hard to tell them apart from the real world… then they’ll presumably make many of them and the odds of us being in the one real world is slim.

Here’s a more elegant way of putting it, from Nick Bostrom:

At least one of the following three propositions must be true:

1. Almost all civilisations at our level of development become extinct before becoming technologically mature.

2. The fraction of technologically mature civilisations that are interested in creating ancestor simulations is almost zero.

3. You are almost certainly living in a computer simulation