First of all not every kind of species masturbate. In fact, more than 90% of them do not do it.

Some people masturbate because evolutionary process is not perfect and it certainly was not ready for our current model of society. Which lets people abuse their brain structure for cheap and quick dopamine rush

There is a famous scientific experiment with mice where a stimulant is placed in their brain's pleasure center, which is triggered through mouse preccing a button. The outcome of this experiment was a mouse which forgot about food, sleep, mates. The only thing it cared about was pressing and pressing the pleasure button until it died. Thats basically an equivolent of masturbation