Small holes in the lower abdominal area are usually caused by a combination of mechanical friction and the structure of modern fabrics—almost never by moths.

The main causes at a glance:

Friction against trouser buttons and belts: The fabric sits right at the level of trouser buttons, rivets, or belt buckles. With every movement, the T-shirt rubs against these hard metal or plastic edges.

Pressure from everyday situations: Leaning frequently against kitchen counters, desks, or tables—or wearing a car seatbelt—compresses the fabric between the hard edge of your trousers and the object. This causes the fine cotton fibers to gradually break.
Delicate yarn quality: Many T-shirts today are made of very thin, fine single-jersey fabric. The short fibers used in these fabrics withstand localized stress less effectively than heavy, tightly woven materials.

The final blow in the washing machine: Everyday wear gradually weakens the fibers. During the spin cycle or through friction against the zippers of other garments, the compromised areas finally tear open completely—making the hole visible.
Tips for prevention:

Wear the front of your T-shirt loosely tucked into your waistband (the “French tuck”) so the fabric doesn’t get pinched.
Choose belts with smooth, rounded buckles.

When buying, look for higher fabric densities (a weight of approx. 180 g/m² or higher) or combed cotton.
Always close zippers and buttons on trousers before washing, and wash delicate shirts in a laundry bag.