Mainly because no past human civilization appears to have had the ability to time-travel.
Which suggests some issues:
1. Must time travelers from Earth's past be human? It's possible to imagine that dinosaurs achieved intelligence and perhaps even science and technology late in their history, only to have it wiped out by some stray asteroid or something. (That idea has appeared repeatedly in science fiction.)
2. Would a technological civilization tens of millions of years ago have left traces for us to discover today? I'm not sure, but I'm inclined to think 'yes', which is my main reason for doubting this speculation.
3. Assuming that it's possible at all, must time-travel require science and technology?. Could it be accomplished some other way? So would it really have required an industrial civilization to accomplish?
With respect to (3), tardigrades could remain dormant for thousands of years. So it would require some kind of (natural evolved) sapient, macroscopic life-form that was as tough as they were, but faced with the challenges of being larger and presumably more complex (i.e., more vulnerabilities to overcome during "hibernation").
And thereby, one thing about "time travel" from the past is that it's actually possible in theory -- from a subjective standpoint at least. And not just in terms of relativistic rockets (like matter/antimatter propelled ones).
If a civilization reached a level where they could engineer self-replicating molecular machines, and the original intelligent organisms had also incrementally replaced themselves with artificial or transbiological bodies...
Then a hidden colony of the microbots (and some larger storage devices they might be responsible for copying/preserving over the same time period) could be programmed to keep maintaining themselves in that fashion over the course of millennia, or even a few million years (recycling their own deteriorated component materials whenever possible).
Upon reaching _X_ date, the colony begins reconstruction of a few now long-dead members of the transbiological creator species ("time-traveling" representatives of that bygone culture), from retained "blueprint instructions" that include the personal memories. Uploading memories and personality configurations from artificial bodies designed with that capacity would be vastly easier to accomplish than with the near impossible(?) challenge of doing that with biological brains.
The colony could even be concealed in the interior of another extra-Earth body (like the Moon). But that presents the additional task of the microbots and larger robots they build having to grow/assemble a new spaceship from the surrounding raw materials. And having to deal with perhaps greater and harsher environmental stresses over the lengthy "time-travel" duration (or not). And solar-power arrays would be visible if they couldn't tap available geothermal energy -- though fusion or nuclear energy would surely be within their prowess, too.
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