Watches routinely outlive several straps, and the failure is usually at the same points. The strap is the only part of the object designed to be replaced.
The strap takes all the movement
A watch case is rigid and static, while the strap flexes with every wrist movement through the day. That is thousands of bending cycles concentrated in a few narrow zones.
Those zones are the lug ends, where the strap wraps around the spring bar, and the buckle holes. Both combine tight bending with concentrated load.
Leather cracks and stitching abrades at exactly those points. The failure location is predictable enough to be diagnostic.
Skin chemistry is hostile to leather
Perspiration is salty and mildly acidic, and it soaks into the underside of a leather strap all day. Body oils add to the load.
Leather tanned for straps is not treated to resist that combination indefinitely. Salts break down the fiber structure and oils displace the finish.
This is why the underside deteriorates first and why a strap can look presentable while being close to failure. The visible side is the protected one.
Metal bracelets fail differently
A bracelet does not crack, but the pins and links wear against each other with the same flexing motion. Worn pin holes produce the stretch that owners notice as sagging.
Stretch cannot be reversed, only reduced by replacing links or pins. The wear is internal and invisible until it accumulates.
Clasp springs are the other common failure, and their consequence is more serious. A failing clasp gives little warning before the watch comes off the wrist.
Spring bars are the weakest component
The thin bar holding the strap to the case carries the entire weight of the watch on two small shoulders. It is inexpensive and it does fail.
Fatigue, corrosion and a knock on a door frame are all sufficient. Most watches lost from a wrist are lost this way rather than through strap breakage.
Replacing spring bars when replacing a strap is cheap and eliminates the most likely single point of failure. It is routinely skipped.
Rotation is the practical remedy
Leather that dries fully between wears lasts substantially longer than leather worn daily. Alternating between two straps roughly halves the exposure of each.
Rubber and synthetic straps handle sweat far better and suit warm weather and exercise. Reserving leather for conditions it tolerates extends its life considerably.
Treating the strap as a consumable rather than as part of the watch changes the calculation. A strap replaced on schedule costs less than a watch recovered from a sidewalk.