If we reverse the question exactly, I’d say that personally, as a secular woman, I wonder exactly the same thing about you. How so many serious, talented, rational people are also religious.
An interesting reflection.

The symmetry is completely clear and natural. Two remarks:
1. Despite the symmetry, a person who has formed a position can still ask why there are smart people on the other side who think differently. The fact that they ask the same about him does not answer his question.
2. There is a difference between the sides. The religious side is more familiar with the secular side than vice versa. Not only sociologically and in terms of content (there is no such thing as secular content; secularity is an absence), but philosophically as well. Very few secular people undertake an in-depth inquiry into the foundations of faith and religious thought. They usually dismiss it and think there’s really nothing there to examine. Religious people usually think they know the secular arguments, and many of them weigh those arguments when forming a position.
This is an important distinction, because if it is correct, then the explanation for the religious person’s question (why are there smart people who are secular) is that they did not seriously examine the religious arguments. But the answer to the parallel question of the secular person (why are there smart people who are religious) is not of that sort.

I’m sure I’ve opened Pandora’s box here, and since these things have already been discussed several times (for example in light of peer disagreement and more), I don’t really have the energy to get into it again.

I didn’t write that just to argue. It really was interesting to see.
It made me think that drives and impulses stand at the base of much belief—religious or atheist.
And afterward people try to find clever rational justifications for it.
By the way, I think the dismissiveness is mutual, not only from secular people toward religious people as you wrote.

Not long ago I heard an argument—in the context of a completely different discussion—from an atheist about the reason for his atheism, and I was shocked by how primitive the argument was. That person is one of the leaders of the Kaplan protest, on its philosophical and ideological side, and I was embarrassed for him. He spoke with great passion, out of deep inner conviction, in much the same way, mutatis mutandis, as you might hear a Breslov Hasid speak in praise of traveling to Uman for Rosh Hashanah—which only made the embarrassment that much deeper.

It’s hard for me to think that one can say about atheist and agnostic philosophers and scientists that they are unfamiliar with the believers’ arguments in favor of the philosophical God (which is what the question was referring to).

I think it’s safe to say that almost all of them have analyzed the cosmological argument and the design argument.
Examples we all know: Einstein, Dawkins, Sagan, Feynman, deGrasse, Daniel Dennett, and also our dear Jeremy Fogel.
Not long ago I also heard Professor Maza from Tel Aviv University say that most of the professors and faculty members in physics at Tel Aviv University do not believe in God. And really, many more examples could be brought.

What am I missing that they aren’t missing?

The question is open to everyone, to Michi and to others.

I once read here on the site that very often those who leave religion are דווקא the best of the youth, those who had the inner integrity to ask questions, and after they did not receive satisfactory answers, they left religion.
In other words, some of today’s secular people definitely did know religion and the alternatives, and chose differently.

I don’t like comparing this to religious youth who leave religion, because many times that stems from hostility toward religion, and it could be that they do remain believers in some sort of God.

I’m trying to focus on the issue of certainty regarding the philosophical God, which is relevant to agnostics who don’t take such certainty for granted, and of course also to atheists, especially those who have thought deeply about the topic (as mentioned, Dawkins, Einstein, etc.).

I don’t have a lot of knowledge about this, but I do know that there were some intellectuals who believed in a philosophical God.
Two examples are:
A. Einstein—he believed in some kind of higher power; he simply did not believe in a religious God. He saw religion as a collection of legends and superstitions.
B. Viktor Frankl—had a positive attitude toward belief in God, but was critical of the interpretation that organized religions give to God.

My guess is that perhaps people are hesitant to admit belief in some kind of God because they fear that the next step will be an attempt to make them religiously observant. The various religions appropriate the idea of God for themselves, and it is hard to stand up against them. It is easier to deny everything.
The idea of “emptiness” or “liberation” in Buddhism דווקא speaks to some people because it is not about religion or belief, but about systematic practice and inner observation.

By the way, in the book Zen and the Art of Motorcycle Maintenance, the author refers to the university institution as the “Church of Reason,” in which faith in rationality replaces religious faith. So perhaps some educated people are afraid to go against the dogma of the institution to which they belong.