on daf33a,Ravina explains that the reisha of the braisa is referring to a tree with a width of 4 tefachim and the seifa to a tree lacking 4 tefachim in width.additionally,Ravina explains that the seifa is per the shita of Rav Yehuda,who held that the eruv be situated on a place 4 tefachim wide.Why doesnt the gemara simply reconcile the braisa as discussing a tree with a width of less than 4 tefachim,so that if the eruv is above 10 tefachim, it is not an eruv since it is not resting on a makom chosuv,as per Rav Yehuda,and when it is under 10 tefachim, it is a valid eruv since it is lying within a reshut harabim,as per the shita of keluta?
Excellent question! Your question is on the Gemara's explanation of the Reisha of the Beraisa. Why does it explain the case as referring to a tree with a width of 4 Tefachim, and therefore the Eruv is invalid when placed in the tree higher than 10 Tefachim? The Gemara should say the case is referring to a tree which is not 4 Tefachim wide, and the Eruv is invalid when placed higher than 10 Tefachim because the Tana is Rebbi who requires an area of 4 Tefachim, and the Eruv is valid when placed below 10 Tefachim because of Kelutah!
The truth is that you could have just as well asked your question on the Gemara's explanation of the Mishnah (32b). Why didn't the Gemara establish that the Mishnah is following the opinion of Rebbi, and that is why it is a valid Eruv under 10 Tefachim but invalid below 10 Tefachim.
Perhaps indeed, once the Gemara explains the Seifa of the Beraisa as going according to Rebbi who requires an area of 4 Tefachim, the Gemara could have explained the Reisha - and the Mishnah - as referring to a width of less than 4 Tefachim as well. That is, both ways of the reading the Beraisa and Mishnah are correct; your way and the Gemara's way, and there is no great advantage to one over the other.
However, as Rav Mordechai Rabin(owitz) pointed out to me, you hit upon a strong question that puzzled a number of Rishonim!
(a) The RITVA and TOSFOS HA'ROSH citing RABEINU YONAH answer that the Gemara did not suggest your explanation since the Beraisa concludes that according to the Chachamim, "any Eruv that cannot be taken [on Shabbos], it is invalid." From this it appears that the main deterrent to the Eruv according to the Rabanan is the prohibition of Meleches Shabbos. Since the Rabanan said " any Eruv...", it would appear that the Rabanan understood that Rebbi, in the Reisha (and the Mishnah too) invalidated an Eruv above 10 Tefachim due to the necessity to perform a Melachah in order to get to it (Hotza'ah).
(b) The TOSFOS HA'ROSH cites his Rebbi, the MAHARAM, who gives another answer. The following Mishnah (34b) discusses an Eruv "on top of a reed, even 100 Amos high" as being valid. The Mishnah therefore cannot be following the opinion of Rebbi Yehudah, who requires 4x4, since the top of a reed is probably not 4x4 Tefachim. (The Reisha of the Beraisa does not bother him for a reason similar to the one that I mentioned, since Rebbi is the author of the Beraisa anyhow and therefore both ways of learning it are true.)
Thank you for your stimulating question,
-Mordecai