Gloves are probably the piece of equipment that live the toughest life. Our hands are constantly engaged while riding, which means that gloves face more wear and tear than a helmet or a pair of pants do. As you grip the handlebar and activate the levers, this puts your gloves through a decent amount of wear.
With time and usage, the materials (leather, cordura, etc.) tend to gradually become thinner, especially on the palm of the hand. The double stitching and the padding slowly wear down and lose their protective thickness and integrity. The stitching can even come undone—it's happened to me! The thinner the material, the less protection it offers in case of a slip and slide.
If you get very little use out of your gloves or have a number of pairs you swap depending on the weather or on your mood, then obviously, a pair will go a long way. According to Ryan F9, you will get roughly 12,000 miles of usage out of a good pair of gloves. There’s no ticking clock for gloves as they usually don’t contain materials that degrade while the gloves are sitting in a drawer.