"And if you do not drive out the inhabitants of the land, then those whom you let remain shall be irritants in your eyes and thorns in your sides" (Numbers 33:55). The Lord said to the children of Israel: "I have commanded you to utterly destroy them, but you have not done so. Instead, you have engaged with the ways of the immoral woman, etc., and you have followed the practices of the wicked beasts." Behold, Jeremiah comes from her children, and he reproaches you with harsh words that displease you, and his prophecies are thorns in your sides. Therefore, the verse needs to specify that these are the words of Jeremiah, son of Chilkiah.
The Warning of Thorns for Those Who Remain
Yalkut Shimoni on Torah 787:6
(במדבר לג נה) וְאִם לֹא תוֹרִישׁוּ אֶת יֹשְׁבֵי הָאָרֶץ. אָמַר הַקָּדוֹשׁ בָּרוּךְ הוּא לְיִשְׂרָאֵל, אֲנִי אָמַרְתִּי (דברים כ, יז) "כִּי הַחֲרֵם תַּחֲרִימֵם", וְאַתֶּם לֹא עֲשִׂיתֶם כֵּן אֶלָּא (יהושע ו, כה) "וְאֶת רָחָב הַזּוֹנָה" וְגוֹ' הֶחֱיָה" וְגוֹ', הֲרֵי יִרְמִיָּה בָּא מִבָּנֶיהָ וְעוֹשֶׂה לָכֶם דְּבָרִים שֶׁלְשִׂכִּים בְּעֵינֵיכֶם וְלִצְנִינִם בְּצִדֵּיכֶם, לְפִיכָךְ צָרִיךְ הַכָּתוּב לוֹמַר "דִּבְרֵי יִרְמִיָּהוּ בֶּן חִלְקִיָּהוּ".