The train of thought that leads to that belief is usually along the lines of: We’re the only sample we have. It’s more likely than not that what our planet and ecosystem has produced is not an outlier but the norm.
That being said, of course I strongly believe those to be fake and also assume that there is a huge amount of variance in what intelligent life with potential to develop spacefaring technology could look like. Therefore we’re probably not an outlier, but the possibilities within non-outliers are still so vast that our first contact would likely look a lot different.