You have to have been living in a bubble not to have at least heard of alien's crash landing in Roswell New Mexico in the summer of 1947. The truth is out there Scully. Well I am not going to argue for or against this account. Sometimes it is better not to know certain things. What I do want to point out is how time affects events if certain things actually did happen. Ready…Set…Go!
If space aliens did crash land in the US in 1947 and they survived then that means over 60 years have passed since it happened. This would make any humanoid life form with a similar human life span very, very old now or most likely dead. If this is the case then any useful information these aliens could have provided is now unattainable. That means that any knowledge of alien technology to be learned has to now be acquired from the recorded information. Also remember that earth Astronauts are not necessarily experts in earth technology there is no reason to believe that alien astronauts would be experts in their own technology either. That would make any records US scientist have of these aliens and their technology at the very best incomplete.
Also given the political climate at the time of the Roswell crash there is no reason to believe that the US military that intercepted the stranded aliens would have treated them as anything other than prisoners of war that invaded US air space. If this is the case then we have to consider the possibility that any information gathered from these aliens would be clouded by the processes used to extract information from these aliens. Over time the methods must have become less direct and colored the way the aliens interacted with the US military. It is even possible that the aliens began to suffer from Stockholm syndrome and began to identify with their captors. Perhaps even leading to human, alien breeding programs.
These stranded aliens must also have broken contact with their base operation eventually leading to rescue or recovery missions which depending on their findings might escalate or placate. It could very well be that any living alien dependents might be half human and not identify with their alien family and might be actively avoiding an alien reunion. If this is the case then this possibility might also have been considered by the alien rescue team and they might be prepared for a large scale confrontation. Finally a space faring civilization with easy access to the universe must also have access to time and dimensional travel which could inevitably mean that we are already surrounded.