Actually, poké balls are quite frequently reusable. It’s just that a number of things may happen in the heat of a battle that render attempts to pick up and try again with the same ball extremely difficult:
1. A trainer may throw the ball too hard and accidentally break it. This occurs more with rock- or steel-type pokémon, or in instances wherein the ball straight-out misses.
2. If by “miss” we mean “fail,” then a pokémon may break the mechanism that generates the capture grid upon breaking free of the ball. (This is the most common reason poké balls are rendered unusable after a capture attempt.)
3. The ball misses and rolls away, to a position that’s not easily accessible by a trainer.
4. The pokémon attacks the ball (after it misses), thus breaking it before continuing to attack you.
5. The ball misses and rolls away, and before a trainer can go after it, the wild pokémon prevents them from doing so, which then causes the trainer to lose track of the ball.
6. The above occurs, but the wild pokémon attempts to flee instead of attack. Depending on the rarity of the pokémon, a trainer may simply forget about their discarded balls in order to go after their attempted catch.
Luckily, in any of these cases, poké balls are rather cheap. This not only means it’s easy to replace them, but … in a way, it’s also an explanation. Common poké balls are made of flimsier material, and thus, it’s not really all that surprising that the most common ways they’re rendered unusable is if they accidentally break due to a pokémon’s efforts or simply by throwing them too hard at a solid surface. The more powerful or expensive a poké ball is (that is, if we’re considering great, ultra, or master balls), the better the materials, of course, but aside from master balls (which are made of the most durable materials but have a nearly flawless capture rate), there’s still the chance that these poké balls’ mechanisms will still fail due to one of the above factors.
The Pokédex According to Bill 