Let's wait and see what proof they have.
There are more stars in the observable universe than grains of sand on all the beaches of the world. When there are countless billions of planets circling countless billions of stars it's inconceivable that planet Earth is the only place in the whole universe where every form of life happens to exist. The odds are so infintessimally remote that life on Earth is alone that we have to say there MUST be life on other planets and in space.
The argument that life can only exist on Earth is now just an echo of the old conceited human view that Earth and we are somehow special and unique. Ours was considered to be the only solar system to exist, then (until the 1920s) the Milky Way was thought the only galaxy in the universe. Humans thought themselves unique as intelligent, toolmakers, builders etc.
All of those conceits have been dismantled over the years until all that's left is the idea that we might be alone in the vastness of the universe. It's the last desperate preserve of those who still want to think humans are somehow special in the universe and won't accept the fact we are not.