This question previously asked why Rashi sometimes says he doesn’t know what a passuk comes to teach us. Both the questioner there and the various sources cited in this answer there seem to assume that it would have been better for Rashi to say nothing rather than ask a question and leave it unanswered.
The Maskil L’Dovid cited there asks the question as:
מה בא ללמדנו שאינו יודע אם לא ידע ישתוק
What is he coming to teach us that he doesn’t know [this]? If he doesn’t know, let him be quiet!
Likewise the Sifsei Chachamim’s wording:
יש מקשים מה בא להודיענו שאינו יודע אם לא ידע ישתוק
There are those who ask what Rashi is coming to inform us that he doesn’t know [this]. If he doesn’t know let him be quiet!
(I’d quote the Lubavitcher Rebbe cited in that answer as well, but I don’t speak Yiddish.)
What is the basis for this? If Rashi’s point in writing this commentary is to explain the Pesukim, why would it be better for him to be quiet rather than alert us to a problem that he can’t answer?
There’s certainly precedent for asking questions and leaving them unanswered. All throughout Shas halachic questions are posed and left unanswered, but there’s what to learn just from the question. Why isn’t that an option here, to explain that Rashi, in his goal to explain the Pesukim, felt that it’s better to highlight a question than leave it unanswered?
Others in the cited question answer similar to what I’ve outlined above. The Lubavitcher Rebbe, for instance, answers that Rashi just didn’t have a sufficiently simple explanation, even if he had a Midrashic one. But the Maskil L’Dovid doesn’t like this, as he poses a wildly different understanding in Rashi, where “I don’t know” doesn’t refer to the explanation of the passuk at all, but rather is its explanation, in a sense. Likewise, the Sifsei Chachamim, in an admittedly simpler explanation, understands that Rashi wasn’t sure which of several explanations were correct, but still isn’t willing to answer that Rashi didn’t have an explanation at all.
Why were these two commentaries unwilling to take Rashi at face value, insisting that it can’t be he said “I don’t know” because if he really didn’t he wouldn’t have said anything?