[Panzerschrek]

This paper doesn't account the possibility of a civilization to spread itself and to fill eventually the whole galaxy. But in such case it's practically impossible for such civilization to extinct. Since we see no intelligent signatures everywhere, it's certain that no such civilization was formed in our galaxy.

[gizajob]

It’s not really possible to say that it is certain given we haven’t looked very hard or for very long. We don’t even know if there’s life in nearby places like Enceladus.

[Panzerschrek]

We don't need to look hard. If interstellar travel is possible and someone in our Galaxy have developed it in the past, the whole Solar System including Earth should be already colonized.

[gizajob]

There are many other answers to why it isn’t outside of “they should have done this but haven’t and therefore cannot exist”.