50 years, oh wow.
100ish years ago there was no evidence of oil below the surface, and now it is such a big part of our lives. 200ish years ago there was no evidence of dinosaurs, 500 years ago there was no evidence of life beyond the landmass of euroasia. Or that the earth was round, or revolved around the sun. There was no evidence of past civilisations, till there was. And so on and on. 50 years is nothing.
Or however long really, is not enough to to dismiss theoretical possibilities of life elsewhere in a univerese as vast and as effectively timeless as this.
The discussion of whether there actually has been evidence and is being concealed is another matter, and should be much easier to determine really. But a 'well if it might exist why haven't we found it in 50 years', is as pointless a take as they get, when discussing evolved life beyond this planet.