>There is a way to know, we exist.

That is not proof that it has happened again, or happened before. I'm a firm believer that life does exist on other planets somewhere in the universe, but without knowing exactly what is required for life to form, the likelihood of abiogenesis, the likelihood that life would form.

Using Drake's equation you could get results as low as 1 advanced civilization per 1 quadrillion galaxies, implying we are likely alone, to as high as millions of civilizations in the Milky Way alone.

Just because something has happened does not mean it's likely to ever happen again. If I won the lottery, it would not prove that I was likely to do so again let alone that I definitively would.

​

>there are millions of species on a single planet

As life on Earth evolved from common beginnings, this does not prove likelihood of intelligence arising (or even life arising.) If anything, it demonstrates how rare the evolution of highly intelligent life is. It took millions of tries for evolution to produce a single highly intelligent lifeform capable of communicating over vast distances and transiting off it's host planet.