With many thousands (perhaps tens of thousands?) of years having passed since these events occurred, I doubt any of us will ever simply work out the true reason for any alien visit in the ancient past; although further clues may be found on Mars shortly?
The basic facts remain however:-
1. The great pyramids exist, as does the artificially levelled plateau/space-port at Nasca.
2. The examples of 'megalithic' stone work at Baalbek, Puma Punku and elsewhere can be seen to be the products of machine cutting/drilling.
3. Recently the amazing revelation that humanity is carrying some two hundred plus unique genes (genes without any track record in other species) which seem concentrated in the development of reason/intelligence and/or vocal ability.
4. Knowledge of electricity (Bagdad batteries), manufacture of aluminium (The Nanjing Belt), brain surgery (trepanning) and much more highly advanced knowledge was known of in the distant past.
5. The Piri Reis Map, the Orontius Finnaeus map and others show Antarctica as it would appear today, if not covered by the current ice-cap.
6. Right around the planet, ancient legends talk of 'Gods' that came down from the Heavens and created mankind.
This cannot all be fraud/hoax and lies; BUT taken together, it's evidence every bit as strong as that Charles Darwin initially brought home on the Beagle almost two hundred years ago; so why isn't today's scientific world as open to new ideas as their forefathers were in the Victorian Era?