Centuries and seemingly unchanged appearances of UFOs

I’ve recently gotten into UFO and NHI sightings/encounters and realized that there doesn’t seem to be much change in appearance of these craft from ancient rock carvings to present day photos. At first, I thought this was odd, because you’d think if these beings were anything like a typical civilization in that the community overall tends towards technological advancement then you’d expect distinctive and noticiable changes in craft. I think of our progress from ancient times to now, with horse and chariot juxtaposed to a Tesla and wonder why NHI civilization’s assumed advancement seems unchanging. Is this because there is a point in which progress levels off, at which all to be known is known and no further advancement is possible? Or could it be due to the passage of time is significantly slowed for them – given the perceived rates of speed they are known to travel at? If this where the case, time would transpire much, much slower relative to earth’s time – a few minutes for them would be decades here on earth. I’m watching Netflix’s encounters and just got me thinking about this and wanted to see if anyone had any similar thoughts or different potential explanations.