Another interpretation: because they are the roofing of the world. One who roofs properly places the thick end of this beam beside the head of another. And how do we know that they are the roofing of the world? As it is said, "Listen to Me, Jacob, and Israel My called one [mekorai]" (Isaiah 48:12).
Israel as the Roofbeams That Hold Up the World
Yalkut Shimoni on Torah 162:8
דָּבָר אַחֵר, לְפִי שֶׁהֵן תִּקְרָתוֹ שֶׁל עוֹלָם, לְמִי שֶׁהוּא מְקָרֶה כָּרָאוּי נוֹתֵן [עֹבְיָה] שֶׁל תִּקְרָה זוֹ בְּצַד רֹאשָׁהּ שֶׁל אַחֶרֶת, וּמִנַּיִן שֶׁהֵן תִּקְרָתוֹ שֶׁל עוֹלָם, שֶׁנֶּאֱמַר (ישעיה מח, יב) "שְׁמַע אֵלַי יַעֲקֹב וְיִשְׂרָאֵל מְקֹרָאִי".