Tri-blends are made of cotton, polyester, and rayon—typically something like 50/25/25. The cotton gives that soft, worn-in feel, the polyester adds strength, and the rayon provides drape. But that combination also means you have three different fiber lengths and strengths twisted together. Short fibers, especially from cotton and rayon, work their way to the surface with every wash and wear. When they hit friction, they don't break cleanly—they curl into balls.

Polyester is actually the strong one here, but it's also the one that holds onto the cotton and rayon pills, making them stick around longer. So the very thing that makes tri-blends comfortable is what makes them pill.