I absolutely agree that they could be from a prior civilization... but they never left... They have always been here. We cannot consider time as linear with this situation.
Given the wide variety of craft seen over the last few centuries and, given that the Earth is 4.5 billion years old, that variety could be representative of multiple prior civilizations.
“We cannot consider time as linear with this situation.”
I take that to mean linear civilization time, one civilization building on the prior one. People today think there were no other civilizations because they are not taught ancient of prehistory, and what they are taught is based on some one’s consensus thinking - (since things evolve, and nothing is happening now we are safe).
From the sheer number of stone monuments all around the Earth - all on some Earthly coordinate system or another, they built structures we cannot duplicate in either construction or architecture or even have reason to do so. They all have some purpose(s) which we are barely understanding: all are built on fault lines or in high lighten strike areas where piezoelectric currents abound. Some like the Giza complex were built as one unit with multiple monuments and mathematical representations. Why?
Asking questions is fraught with danger: just doubt the Ashli Babbitt story and run.