We've never seen a star form. We have never watched an alpha particle leave an atomic nucleus. We have never even seen an electron. We have never seen an underground magma pocket. We have never seen the iron cores of the Earth or the Moon.
Some quacks like to insist that, if we haven't watched an event unfold with our human eyes, then we cannot find good basis to know it happened. Of course this is utter nonsense, and we would still be trying to figure out what causes syphilis, were this the case: Demons, or god's wrath? Or witches?
Just think if murderers could use, in their defense, the idea put forward by these quacks. "You can't PROVE my DNA was not deposited at the crime scene or even at your lab by magical fairies!"
Can you imagine them as kids? "Son, I told you no more chocolate, yet there you are with chocolate on your face." "Prove a magical fairy didnt put it there! Prove it has not always been there! You didn't see me eat chocolate!" .... Haha...total nuttery....
This quackery is a stupid talking point put forth by young earth creationists and is reserved only for the science that contradicts their strident dogma. They don't seem to mind the known half lifes of isotopes when they need radiation therapy for cancer; but use that knowledge to determine a fossil is 60 million years old, and suddenly it's time to pull out the "you dont know, because you weren't THERE, man!!!" argument.
An evolutionary microbiologist determines which strain of a particular bacteria is likely infecting their loved one, and they seize on the knowledge to help their loved one. Remind them that this knowledge was determined by assessing 100s of 1000s of years of evolution of the species, and they will bring out the fairies and sky daddies again.
Use our knowledge of mitochondrial DNA to find better organ donor matches, and the quacks are eternally grateful for saving the lives of their loved ones. Use that same knowledge to determine that two populations of a species have been separated for 1,000,000 years, and here come the sky daddies and the "you weren't there!" nonsense.
And abiogenesis? Their heads explode. Apparently, god can do anything and everything...but he becomes Meatloaf, when it comes to abiogenesis. "But I won't do that!" Funny thing is, abiogenesis is what the bible describes. Life, from "not life". So apparently, god could and would "do that". But no, he wouldn't. Or wait, yes he would. Intellectual fumduckery....