The trouble is that we have about the same, or in your terminology "not one good reason" to imagine that the alternative scenario is true either. If G and not-G are equivalent scenarios with little or no reason to believe or disbelieve either of them, then they are about equally plausible and we should have about 50% confidence that G is the case, and about 50% confidence that not-G is the case.
The complication arises in determining equivalence between those variables: The more specific G becomes, the less it is equivalent to not-G and therefore the less plausible it must be, all else being equal. For example, "Brad Pitt ate a meal today" (a) is more probably true than "Brad Pitt ate spaghetti today" (b) which in turn is more probably true than "Brad Pitt ate spaghetti at Richard Norton's house today" (c). C is a subset of B (alongside spaghetti with David Fincher, spaghetti with Morgan Freeman etc.), and B in turn is a subset of A (alongside a meal of pizza etc. etc.), so the share of probability or conceptual space that C occupies must necessarily be no larger than and all else being equal will usually be much smaller than that which A occupies.
It's only when pared down to a single and apparently fundamental or irreducible attribute - consciousness, the single most certain thing we can know and also one of the great enduring mysteries of science - that we'll be left with a genuine equivalence between G and not-G: Either reality is fundamentally conscious or it is not, and we genuinely have no conclusive way of differentiating between those two possibilities. The former (that reality is fundamentally conscious) has the edge and indeed by many atheists' reasoning, not my own, it should be treated as the 'default' presumption with the alternative laughed out of consideration, since lacking the ability to detect the presence or absence of consciousness we have "no evidence" that non-conscious stuff even exists at all! Indeed there are other, more reasonable psychological and developmental explanations for why we invented this notion of 'non-conscious' stuff, and introducing the notion of non-conscious stuff adds more questions than it answers, reducing parsimony and 'violating' Occam's Razor... so it's actually a pretty substantial edge that the 'reality is fundamentally conscious' view ends up having.
Regardless of whether it's ~60:40 (my ballpark) that reality is fundamentally conscious or ~100:0 (atheist reasoning, if they applied it consistently without special pleading) that reality is fundamentally conscious, we'd also have to note that "reality is fundamentally conscious" isn't strictly synonymous with theism. Some ways of thinking about panpsychism might be non-theistic alternatives to pantheism or panentheism, for example. But it's a pretty solid starting point for an agnostic ~20-80% range of confidence or speculation that the nature of our reality may indeed be some kind of god.