1. They are experimenting on us
Possible they are altering us for different types of experiments some of these are pretty dark Wars, diseases etc could in fact be tests for various types of test in order to bring in field test results, we may not understand it, but it could be bring in their desired results.
The best way to get target data for a weapon for example would be to use most accurate specimen and environment of the target in question the weapon was designed for.
this would support the ancient alien astronaut theory,conclusion tho would be at the very least we would be genetically identical to the enemy of who they are creating the weapon to be used on.
2. They are recon us for a invasion
Not invasion as it would be unlikely would of happened ages ago, but I suspect there are worse things than war or death, they could be planning that are beyond our understanding, that they could be planing, other possible is the invasion already happened, with their intelligence they would very understand and fully understand the ideals of not having to destroy something to control or achieve their goals they wish, benefits of such would be the enslaved race would not uprising as they are unaware of being enslaved
3. They are from the future
This would also explain actions of our government i.e. they have been sort of informed or figured this out themselves, informing the population would perhaps effect their future in ways they do not want, our governments behaving in a way that implies such thing, they are building bunkers are highest rate ever in history, stockpiling water, resources also at a high rate, the aliens could be global elites dissidents ,the future humans could be here to make sure this all happens and monitor everything, few conclusions to this theory that sort of support it, one thing is the tech these entities are using is not nearly as advanced as it should be, if life form has been visiting us since before our race became intelligent it would be far more advanced then the entities has been demonstrating ,the tech they have been demonstrating is maybe 100 to 200 years in front of our own tech,this means one of the following things ,i.e. they failed in their goal or achieved it before they advanced to a higher level and never visit or need to returning to this time ever again, but I suspect there would be some encounters so it likely means they likely failed ,there have been stories of possible more advanced tech but it is more likely they are not as far ahead of us as some believe, some people believe this beings are preparing us to help us suspect this is not the case the humans they have been breeding I suspect are for a future time, not for this present time, the people of this time in their eyes ,
Our fates are is already decided. they have been focused not letting any information regarding their mission out, all people who have been in contact with them ,are told little, only need to know, in order to achieve goal or results, This is in my mind the most likely answer they have most likely told the governments the elite people that going into hiding in bunkers will save them and keeping it secret as they will become them, this is most likely a lie the real asters of these entities are ones they are staying very far away from, they do not want to change there pace in resulting a certain future as it will change themselves in the future, there goal is to change their future not our future, as changing our future will maybe result in unwanted changes to their own selves or worse, what we can conclude is this the future environment they come from is highly changed from this, it has triggered rapid evolution due to their appearance this only seems to happen when creatures are exposed to extreme environment change, rapid evolution helps species to survive this, the humans of this time have changed much from home sapiens question is what triggered the environment change,that has brought on the rapid evolution ?
4 they are passing by/other reasons
but they are watching us due to actions of our government back during the 1950 in taking down one of their craft (which I suspect was due to a nuke intentional or intentional) in this version we are just a stop off, on a route somewhere, we have been noted as slight hazard, there response has been to monitor military installations, obtain full control over warheads, in order to avoid repeat incidents
Other possible is they have no interested in us at all we catch glimpses of them while they pass us, it would be like us getting involved in a argument among monkeys over bananas or something, but also more importantly informing a lower less evolved race maybe a sort of information leak, if this race was discovered conquer by other aliens, or obtained information regarding the said alien race, the information could fall into the hands of a undesirable race, that could be hostile or worse, a good comparison would be animals in the jungle, it is best to keep quiet most of the time, due to the fact tigers etc could hear, and be drawn to your laceration ,this is maybe the biggest reason the universe seems so quiet, contact between races is only made when there is a good, worthy reason, this is not always the case in the jungle there are parts that are very noisy with life, but the fact this part of the universe ether means we are in a remote location or there are predators nearby