Why are quarks never seen alone?

Quarks never appear alone because the strong force's field grips itself, so the field between two quarks stays in a narrow tube whose cost grows with its length. The strong force acts on three slots called colors, its version of charge, through eight independent moves, and its ripples, the gluons, carry color themselves. A ripple that grips its own kind cannot spread out and thin. Pull a quark out of a proton and the tube's cost builds until it is spent making new particles, long before the quark gets free. Gluons are never seen alone either, and colliders see them as jets, sprays of new particles along the direction they left in. Up close the grip slackens, which is why quarks rattle about almost freely inside a proton.

Also in Chapter 27: Why This Exact List of Matter, and Why Three Times Over?.