I mean none of us have actually witnessed them, and none of us actually know anybody who personally experienced them.
What is to stop people who lived 200+ years ago from fictitiously making up civilisations like the ancient Egyptians, or the Persian/Bablyonian empire.
I get that there are artefacts and literature but how do we know that these are authentic and actually from the era and not made recently in a fictitious manner?
Short answer: because science.
Longer answer: we can use various dating techniques to find out how old things are. So, say, we can test a supposed ancient artifact to see if its ancient or modern. There's too many techniques to list in detail (Carbon- dating, presence or absence of various isotopes such as, e.g., fallout from modern atomic testing, and others). Perhaps an expert will be able to weigh in in detail.
But the short answer is we can tell if something dates from the supposed period and often area as it claims to be.
A real-life example: we know the Shroud of Turin is a medieval fake and not the actual burial cloth of Jesus.
On top of that we use common sense. If there is only one source attesting to, say, some supposed ancient civilization and we've been unable to locate any artifacts or other sources that refer to it, then we should be skeptical. But the existence of, say, the ancient Babylonians is attested to by countless artifacts, numerous literary works, ruins and more. That civilization existed.