In mishna Keilim 26:3, we see that a leather belt is susceptible to tumah.
כַּף לוֹקְטֵי קוֹצִים, טְהוֹרָה. הַזּוֹן וְהַבִּרְכְּיָר, טְמֵאִין. וְהַשַּׁרְווּלִים, טְמֵאִין. וְהַפְּרַקְלִימִין, טְהוֹרִין. וְכָל בֵּית אֶצְבָּעוֹת, טְהוֹרוֹת, חוּץ מִשֶּׁל קַיָּצִין, מִפְּנֵי שֶׁהִיא מְקַבֶּלֶת אֶת הָאוֹג. נִקְרְעָה, אִם אֵינָהּ מְקַבֶּלֶת אֶת רֹב הָאוֹג, טְהוֹרָה:
The hand-coverings of thorn-pickers are pure. A belt and leg guards are susceptible to uncleanness. Sleeves are susceptible to uncleanness. But hand-coverings are pure. All finger-coverings are pure except that of fig-pickers, since the latter holds the sumac berries. If it was torn, if it cannot hold the greater part of a sumac berry it is pure. [from Sefaria]
Why should this be the case? Indeed, a flat leather object that isn't made for sitting/lying (such as a belt) is only susceptible to tumah if it is recognisable as a utensil and serves both a person and their objects (as described in, for example, Kehati's introduction to the masechet). While a belt is a recognisable utensil, it serves a person, but not their utensils.
So, why is a leather belt susceptible to tumah?