I think most mathematicians are uninterested in PoM because their interest in math comes from an aesthetic impulse and a problem-solving drive. In short, they like math because it’s beautiful and challenging, and challenging problems are fun for them to solve.

They may also be motivated by prestige, to varying degrees, though I think mathematicians are fairly low on the prestige-valuation scale (viz. Grigori Perelman).

Though I enjoy PoM myself, it actually feels a bit weird to wonder why mathematicians don’t care about it. To me, it’s like wondering why chess grandmasters don’t seem to care that chess is a fictional game and not real warfare.