Explanations

Poul Anderson, The Boat Of A Million Years (London, 1991).

Sometimes a work of science fiction ends by explaining why the world remains unchanged despite a fictional invention or discovery.

Thus:

time travel has not been confirmed because the Time Traveler never returned;

there is no interplanetary travel because Cavor remained on the Moon and the Cavorite sphere was lost;

the Mongols did not conquer North America because the Time Patrol prevented them;

Boat presents an excellent example -

"There is simply a radius, on the order of a light-century or two, beyond which it is unprofitable to search farther. Having foreseen this, you have never built self-multiplying von Neumann machines." (p. 596)

So, yes, there are extrasolar intelligences but, no, no robotic explorers from their civilizations have ever entered the Solar System. Starfarers, to which I will return shortly, describes von Neumanns mutating and thus never completing their missions.