The infinite monkey theorem states that if you have an infinite number of monkeys banging away at random on an infinite number of typewriters one of them will eventually reproduce all of the works of Shakespear (along with every other possible combination of keys). Earth could be that monkey.
The evolution of even the simplest living organisms could be the result of an infinite number of occurrences and conditions, all happening in the exact right order, most of which we cannot even imagine. We could very well be all the life that there is in the universe.
Alternatively, it could be that life exists just about anywhere there is a planet with liquid water, evolving from complex organic molecules through means that we just have not been able to figure out yet.
As I noted above, unless and until we either find life somewhere else or figure out how to create it we just don’t have enough data to know whether life is common or extremely rare.