Maybe the flight to Earth took 15+ years or thousands of years

Depending on which planet they were coming from. Earlier today I looked up how long it would take to get to the hypothetical planet 9 / planet x and one of the estimates was 20 years. So perhaps, just like we're vigorously looking for information on a potential "Planet 9," the inhabitants of that planet have been desperately trying to find out about us? Or, if the UFO was from Proxima B, perhaps their flight here took 70,000+ years.

Assuming they're more advanced, but not advanced enough to travel using teleportation / wormholes, they sent off their astronauts a long time ago. This might explain a potential crash. Perhaps the astronauts were already dead due to some sort of malfunction on the ship (cryo malfunction)??

Maybe only one ship crashed, but they've had multiple successful landings thousands of years ago (which could explain the "ancient alien" phenomena). Maybe we are their descendants. 😮🤔🤯