Dawkins essentially argues that if one observes some event that seems to be designed because it seems very improbable to have occurred, positing God as a hypothesis doesn't make sense, since God is more complex, and given His nature, He seems to be the most improbable hypothesis ever.
But in the case of designed events, those events have a beginning. There was a time that we can point out where that event did not exist, such as the case of life to have arised by chance. There was a time where life did not exist. But we can't say that about God since many conceptions of God are posited to be eternal. And if God is eternal, how does it make sense to say that He is the most improbable when He never really arose by chance?
Secondly, if God is a hypothesis that we haven't actually confirmed to exist, how can we say that He is improbable instead of simply saying we don't know? For example, if He is a necessary being, then by definition He is not the most improbable but rather has a probability of one.
I wanted to illustrate this with an example. Suppose one has an event occur in their life that seems to be a sign from God. Imagine if someone has a predictive dream where in that dream God appears and says "You will see your missing daughter tomorrow". You then see your missing daughter tomorrow.
Let's say you're deciding between two hypotheses that produced that dream: chance and God. There may be other possible hypotheses, but let's assume for the purposes of this particular hypothetical, we're just comparing the likelihood of these two. Now, the probability of you having that particular dream may be very low by chance. One however might say that God sending this down as a sign is even more improbable, since through Dawkin's logic, God's very existence is the most improbable of improbable events. One can then say it's more rational to believe that the dream occurred by chance. But why? What basis do we have of assuming this?
Of course, if God doesn't exist, then it quite literally is impossible for Him to send down a sign. In that case, the dream must have occurred by chance or some other hypothesis. But if He does exist, then how can we know His probability of sending down a dream? More importantly, we don't know if He exists, and hence, we don't know P(God sending down this sign).
If we don't know if He exists, how can we say that P(God sending down this sign) is > or < P(predictive dream occurring by chance)? Also, given that these are binary events (i.e. God either sent down a sign or He didn't; this happened by chance or it didn't), can we even assign probabilities here? Ultimately, that P value seems to be either 0 or 1 for both hypotheses.