This is based on the exchange in Devarim 6:
The pashut peshat of this is that this is not asked specifically regarding the korban Pesach, while the Pesach, matzah umamor are munachim lefanav. Rather, the father is keeping all the mitzvot, edot and chukim. The son asks what they are, not wishing to know the details, but rather to know the import. And the father's response, in context, is precisely to explain the import -- that it is righteousness unto us, and so that Hashem may preserve us alive.
But this is a derasha, and so the pasuk is taken from its simple context and used to speak about another son, at another time. He is transported in time and space to the night of the korban Pesach. And he is not asking about the import, but rather wishes to know about the details of the mitzvot. And the father's response is indeed to inform him of all the details pertaining to Pesach, (including) that one should not eat a dessert after the korban Pesach.
What happened to sippur yetzias Mitzrayim?! These are laws, not story!
One answer is, perhaps, that the response would not only include the halachot, but also some minimal element of retelling the story of the Exodus. After all, in the pesukim above, in context in Devarim, the father responds by telling him Avadim Hayinu, which is Shmuel's beginning, and which in fact starts off all of maggid. But I do not get the sense that this is truly so. For this is a derasha, which is hyperliteral, and takes the words not only out of context but out of meaning. And this son is not asking for import, but details, and the response is the details. The son who asks about the import is more correctly the Tam.
The answer might be somewhat elitist. I cannot help but think of Rav Sheshet, who would turn his back to the leining of Kriat HaTorah. From Berachot 8a:
R. Shesheth used to turn his face to another side and study. He said: We [are busy] with ours, and they [are busy] with theirs.Perhaps only the Tam, whom the Yerushalmi labels the Tipesh, should get the Sippur Yetziat Mitzrayim. But people who are Chachamim, Nevonim, Yodeim et HaTorah should not bother with this, but should instead focus on the Hilchot HaPesach!
To counter this, we have the incident with Rabbi Eliezer, Rabbi Yehoshua, Rabbi Eleazar ben Azariah, Rabbi Akiva and Rabbi Tarfon who were feasting in Bnei Brak. They fulfilled the instruction immediately preceding that even for such people, it is a mitzvah lesapper biytzias mitzrayim, and that whoever increases in this, it is meritorious. And these great Rabbis engaged in sippur yetzias mitzrayim all that night, until the students came and told them that the time for Shema of Shacharit had arrived. Clearly, they held that one should indeed engage in sippur yetzias Mitzrayim.
However, I think I can raise a counterexample. While those chachamim, navonim, etc. were feasting in Bnei Brak, something else was happening in Lod. Citing the last Tosefta in Pesachim:
חייב אדם [לעסוק בהלכות הפסח] כל הלילה אפילו בינו לבין בנו אפילו בינו לבין עצמו אפילו בינו לבין תלמידו מעשה ברבן גמליאל וזקנים שהיו מסובין בבית ביתוס בן זונין בלוד והיו [עסוקין בהלכות הפסח] כל הלילה עד קרות הגבר הגביהו מלפניהם ונועדו והלכו [להן] לבית המדרש
The obligation described here is not sippur yetzias Mitzrayim, but rather to engage in the hilchot haPesach. This might be like Rav Sheshet, that Talmidei Chachamim engage in their own, and the common folk, Tam, engage in their own. And this might be the import of this maamar of the four sons. (Unless somehow the hilchot haPesach are considered sippur.) And whether this is indeed so is a machlokes, and thus it was necessary to state that even kulanu chachamim we have an obligation, and that we should go on at length in this sippur.
I would note that this incident in Lod involved Rabban Gamliel and the Zekeinim. But we know that the same Rabban Gamliel did not eliminate the obligation of sippur entirely. As we cite him at the end of maggid, whoever does not say and explain the import of these three things -- Pesach, Matzah, and Maror has not fulfilled his obligation of sippur. These points would likely have been covered in the course of the seder. Though not necessarily the association of the physical symbols to the messages. But perhaps he has this short list not as a checklist, but because otherwise someone might not cover these points. After all, one might be engaged in discussion of the mitzvos of the night rather than on the story.
Looking now at the Mechilta, it is actually explicit. We have Rabbi Eliezer (one of the Bnei Brak participants, though) stating:
מה העדות - ר' אליעזר אומר:
מנין אתה אומר שאם היתה חבורה של חכמים או של תלמידים שצריכים לעסוק בהלכות פסח עד חצות?
לכך נאמר: מה העדות וגו':
Specifically if they are a chabura of Chachamim or Talmidei Chachamim, this is what they should focus upon. Although this is ad chatzos. (Presumably, since they also have an obligation to eat matzah and drink the four cups, this "maggid" was happening simultaneous to all the other mitzvos halayla.) Though this might be after a regular maggid.
The pasuk in Devarim is
That is, "us", not you. This is not to claim that our Masoretic text is necessarily wrong. Rather, the claim is that for the original Midrashic author, the derasha was on a non-Masoretic text (something we have already seen once or twice elsewhere on parshablog). And so, the question everybody asked would not have even been a question in the first place for the midrashic author.
Of course, I am not convinced that even with our text, the question arises to the level of question. Because we are engaged in derash here, and the midrashic author is entitled to place stress on any portion of any verse he chooses, to bring out his point, without needing to worry about consistency.