There is a very reasonable explanation for this: If we are a topic of research for them, they could have simply stopped studying us in the same way

Take our own science for example. We pull out of studies when the funding dries up. Maybe the aliens’ government grant ran out. Or, perhaps they have a policy of avoiding interference with the subjects. They could have changed methodology in response to the threat of high resolution recording equipment

Yeah there is a reason why plane spotters use 400+mm lenses