The reasons are very complex. So much so that none of us will ever truly understand.
I do not have time to go into this but I would suggest you look into other topics on this board.
When you try and think "logically" of the ET and abduction phenomenon. Just remember your perception of "logic" is very limited in regards to the scope of the universe.
I will only answer what I know, even if it is not believed.
1)yes the governments of the world and intelligence agencies have been covering it up for a VERY long time.
2) Occult initiates are no stranger to intelligences of other realms/dimensions/beings, and many (like Crowley and John Dee) have interacted with them physically.
3)In the intelligence communities at certain levels, yes there is interaction with other beings. And yes 'deals' were also made a very long time ago.
4) Some beings are dying and something we carry/have might of been viewed as part of the solution to their deficiencies.
5) Inside conspiracy theory is a great deal of truth. It is mired by propaganda, disinfo, and the general uneducated guesses of most of those trying to understand information that transcends anything our brain can even process.