You're making assumptions based on logic that might not apply to UFOs, so comparing them to those two hoaxes, would be pointless.
First of all, Big Foot and the Loch Ness monster are, if they exist, Earth based entities to which the physical laws of Earth apply.
They would have to be somewhere on Earth, taking up space at all times.
It would be reasonable then, to assume that if they really exist, better evidence would have been found by now.
With UFOs OTOH, we're talking about objects and/or beings from other worlds and possibly other universes, time/dimensions etc., which can seemingly appear out of nowhere and disappear into thin air.
They might possess technology that enables them to render themselves invisible to the five human senses and human recording devices, while operating right under our noses.
Because we know nothing about what lies beyond the miniscule portion of the universe we are able to observe, all we can do is guess and make assumptions.
Again, the UFO conspiracy theory seems to ask people to believe an extraordinary perfect storm of conditions to actually make alien spaceships a real possibility.
A vast decades-long global conspiracy of silence by the governments of all developed countries.
Exotic technology that can not only travel the galaxy, but can disappear and appear at will.
The lack of any large body of reliable and convincing video footage, despite the daily presence of six billion smart phones on the planet.
An ask for people to accept second hand claims, despite the lack of any verifiable physical evidence.
My basic rule of thumb is that extraordinary claims require extraordinary evidence.
