Because a rock is still a rock regardless of if humans are around to observe it.
Nice, the old "argumentum ad lapidem".
I understand we find it easy, in fact, it's intuitive and downright "obvious" that "things" "exist" "independent of observation and evidence".
But what you seem to be missing is that this "common sense" (a priori "fact") is a de facto philosophy in-and-of-itself, namely Naive Objectivism (naive realism).
Any "thing" we don't have evidence of is an aspect of NOUMENON.
Part of the NOUMENON is potentially "knowable", Mysterium Invisus and part of it is fundamentally unknowable, Magnum Mysterium.
I'd say that "existence" is probably not the best word to describe noumenon (mainly because the definition of "exists" requires empirical verifiability). I believe it is a mistake to imagine noumenon as some sort of "thing" when it is merely an amorphous concept that acts as a place-holder for both "what we don't currently know" (Mysterium Invisus) and "what may be fundamentally unknowable" (Magnum Mysterium). For example, noumenon might be eleventy-trillion layers of sci-fi multiverse, noumenon might be an elaborate alien computer simulation, noumenon might be Brahma's dream, noumenon might be a single super-intelligent (but not omniscient) demiurge that we humans are merely appendages of. In all likelihood, it is conceptually, literally, ultimately and completely beyond our ability to comprehend. All of this makes it very very very difficult for me to believe that we can consider (with any degree of confidence whatsoever) that noumenon is itself comprised of 100% pure, uncut, "objective reality". I mean since noumenon may involve a great many (likely) possibly subjective layers (simulation/dream/multiverse) below our primitive perceptions, although we can deduce with the confidence afforded us by our logic, that there must be, at some level, "real" and "true" and "objective" "reality", we cannot have any confidence that what we are able to perceive has anything-at-all to do with the-hypothetical-objective-essence directly. It's like the old story of the princess and the pea. Clearly there is "something" under the bed, but what are the chances that a normal person would be able to detect it through ninety-nine high-quality mattresses(?).