Because there are a lot of other things necessary for life. Usually we get the 'planet X may be suitable for life' PR news item first, and later in the scientific press we find out quietly 'planet X is awash in deadly gamma radiation, and the tectonics are such that there's a richter scale 13 quake every week'.

It turns out that when taking into account all known factors, us being alone is a nontrivial possibility:

https://arxiv.org/pdf/1806.02404.pdf