If you walk in the dessert, and suddenly find a ticking clock, showing the time and date correctly.
Quick question; how would one know whether or not the time displayed was accurate? More to the point, why would it matter if it were or not?
Besides, the concept of a 'clock' and 'clockness' is an abstracted subjective one, based on societal-ascribed form and function. One would not identify such an object as a 'clock' unless one had previous experience of 'clocks', both as how they were and how they were constructed. In other words (and for more reasons then this), it's a bad analogy.
Show a sundial to a Martian, or indeed a cricket bat to an American, and polite befuddlement would inevitably ensue.
Would you beleive that
1) that a company had created it, a consumer bought it, and somehow it was left, lost, thrown away or something, in the dessert
or
2) that it appeared in the dessert after 1000's of years of different processes that was carried on in the dessert, with combination of lightening, pressure, heat and so on?
Neither, actually. Had I been wandering aimlessly for some time within a desert devoid of all other signs of human existence (as is implied by your scenario), and then miraculously stumbled across this one (perfectly working) aspect of civilisation; I'd seriously doubt my ability to perceive things accurately and honestly, and presume insanity.
So why do the same people say that the solar system was not created? But just happened to be?
Firstly, few people of any religious/philosophical/scientific variation believe that the solar system, or anything else, "just happened to be".
Secondly, there is no compelling and inherent evidence to support the assertion that the solar system was intentionally constructed, whilst there is quite a bit for the assertion that clocks were, both in their actual and conceptual existence. Given that we do live in a finite universe, we cannot consistently proceed from the position that an assertion is true until proven false, as otherwise we'd have to accept the existence of practically anything that could be imagined. Doing so might make things a little cramped.
I have to wonder why the clock is always in the desert. Why can't it be in a tropical rainforest? A nice temperate forest? Taiga?
I suspect it's at least partly intended to emphasise the creative power of a/the God(s), in order to make it appear more remarkable and seeming self-evident. A rainforest would field a whole host of aesthetic and logical distractions for such a hypothetical.