Though we have been looking for signs of intelligent life for ~50 years or so, we have only extremely recently obtained the ability to observe some exoplanets.
Very soon, we will have the ability to analyse the atmosphere of exoplanets for signatures of life such as seasonal methane and oxygen cycles. So till we actually have the ability to look for signatures for simple organisms, we cannot use the fact we haven't found any as evidence against their existence.
Though we currently have not observed a process by which ribosomes can form naturally, this does not mean there is not one that can occur fairly easily. We have only been studying the conditions that were present on ancient earth for a century or so. Nature had hundreds of millions of years.
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