Why is the Gemara using tautology?

Question

The Gemara in Masechet Megillah says:

אֶלָּא שֶׁהַכְּפָרִים מַקְדִּימִין לְיוֹם הַכְּנִיסָה׃ א״ר חֲנִינָא חֲכָמִים הקילו עַל הכפרים לִהְיוֹת מַקְדִּימִין לְיוֹם הַכְּנִיסָה כְּדֵי שיספקו מַיִם וּמָזוֹן לַאֲחֵיהֶם שבכרכין למימרא דתקנתא דכרכין הוי והתנן חל להיות בשני כפרים ועיירות גדולות קורין בו ביום ואם איתא ליקדמו ליום הכניסה הוו להו עשרה ועשרה לא תקינו רבנן
[The Gemara continues with the next piece of the Mishnah:] The villages, however, may sometimes bring the reading forward to the court day. Rabbi Chanina said: The Sages were lenient on the villages to allow them to bring the reading forward to the court day so that they can supply food and water to their brothers in the cities.Is this to say that it's for the benefit of the cities? but it was taught in the Mishnah: If Purim falls on Monday, the villages and large towns read on that day. And if it's as you say, they should bring the reading forward to the previous court day! This would bring it to the 10th, and the Sages did not enact the 10th as a possible day.

TL;DR: The Gemara asks, "Why can't we do this?" and it answers "Because the rabbis didn't establish it.

My question is, is that the question is "Why did the Rabbis not allow this?" and the answer is "It's because they didn't allow it." But the question is WHY the rabbis didn't allow it so this answer is not very satisfactory. IN summary, my 2 questions are 1)Why did the Gemara use such flimsy logic? 2) What's the real reason why the Rabbis didn't allow it?

Answer 1

Your question is asked in the sefer Beis Yitzchak. He answers that the original takanah was not about reading on Monday and Thursday, because the decree that the courts should sit on Monday and Thursday hadn't been decreed yet. That decree came later in the time of Ezra. Rather, the sages in the time of Mordechai decreed that the courts of later generations could institute to read the Megillah earlier if they saw the need, but only up to the eleventh.

Now you really run into the same problem in the next step of the gemara, so see what he answers there.

A simpler approach is that sometimes the gemara says things that could be disproven in multiple ways, but only uses one way to disprove them. What matters is that at the end of the day we get to the truth. In fact, see Rashi earlier who makes the same point. Here also, your point may be valid, but the gemara would rather use a more direct proof. See the Ritva to your gemara who brings another way the gemara could have proved its point in one step.