What are all the smart people missing?

To put the question in a more precise way, I would say that personally, as a secular person, I wonder exactly the same thing about you. How come so many serious, competent, rational people are also religious?
Interesting reflection.

The symmetry is completely clear and natural. Two comments:
1. Despite the symmetry, a person who has formulated a position can ask why there are scholars on the other side who think differently. The fact that they also ask the same question about him does not give him an answer.
2. There is a difference between the sides. The religious side knows the secular side better than the opposite. Not only sociologically and programmatically (there is no such thing as secular content. Secularism is the absence), but also philosophically. Very few secularists enter into a deep investigation into the foundation of religious faith and thinking. They usually disdain it and think that there is really nothing to examine here. Religious thinkers usually know the secular arguments and many of them consider them when formulating a position.
This is an important distinction, because if it is correct, then the explanation for the religious person's question (why are there scholars who are secular) is because they have not seriously examined the religious arguments. But the answer to the secularist's equivalent question (why are there sages who are religious) is not like that.

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

I didn't write just to argue. It was really interesting to see.
Made me think that credulity is at the basis of much of the belief - religious or atheist.
And then they try to find sensible rational reasons for it.
By the way, I think the contempt is mutual, not just of secular people towards religious people as you wrote.

I recently heard an argument – in the context of a completely different discussion – by an atheist about the reason for his atheism, and I was horrified by the primitiveness of the argument. That man is one of the leaders of the Kaplan protest, from the philosophical and ideological side of it, and I was ashamed for him. His words were spoken with great passion, from a deep inner conviction, as, in contrast, you would hear a Breslov Hasid speak in praise of the trip to Uman for Rosh Hashanah, which added great depth to the shame.

I find it hard to believe that atheist and agnostic philosophers and scientists are unaware of the arguments of those who believe in the philosophical God (which is what the question was about).

I think it is safe to say that almost all of them have analyzed the cosmological and design arguments.
Examples that we all know - Einstein, Dawkins, Sagan, Feynman, de Grasse, Daniel Dennett, and also our dear Jeremy Fogel.
I also recently heard Professor Maza from the University of Thessaly say that most of the professors and physics faculty at Thessaly do not believe in God. And I could really give many more examples.

What am I missing that they do not miss?

The question is open to everyone, to Mikhi and others

I once read here on the site that many times those who come out with questions are actually the best of the youth, those who had the inner integrity to ask questions, and after not receiving proper answers, left the religion.
That is, some of the secularists today definitely knew religion and the alternatives, and chose differently.

I don't like to compare with religious youth who come out with the question, because many times it stems from anti-religion, and it is possible that they do remain believers in some god.

I try to focus on the matter of the certainty of the philosophical god, which is relevant to agnostics who do not take this certainty for granted and of course also to atheists, and certainly to those who have thought about the subject (as mentioned, Dawkins, Einstein, etc.)

I don't know much about it, but I do know that there are some intellectuals who believed in a philosophical God.
Two examples are:
A. Einstein - believed in some 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 towards belief in God, but was critical of the interpretation that established religions give to God.

My hypothesis is that people may be reluctant to admit belief in any God, because they fear that the next step will be an attempt to convert them. The various religions appropriate the idea of God for themselves, and it is difficult to stand up to them. It is easier to deny everything.
The idea of “emptiness” or “liberation” in Buddhism, actually speaks to some people because it is not about religion or belief, but about systematic practice and introspection.

By the way, in the book “Zen and the Art of Motorcycle Maintenance” the author calls the university institution a ”church of reason”, where belief in rationality replaces religious faith. That is perhaps why some educated people are afraid to go against the example of the institution to which they belong.