Just how do you know this with such certainty?Conspiratorially - they can exist.
In the 'conspiratorial world' things
- have no mass,
- occupy no volume and
- can be moved effortlessy and
- thereby can be stored anywhere as they
- don't require hangars, garages, bays or tarmac space
The 'conspiratorial world' also
- doesn't require trained personnel,
- food for non-existent personnel,
- sleeping quarters for non-existent nor
- entertainment or recreational diversions for said non-existent personnel ...
So, conspiratorially - they can exist.
Realistically, they are assets; personnel, material, property, money, gear/equipment (incl lab gear) and 'activity' - that 'give things away' to those who know what to look for.