Pirates are by no means a unified faction. They’re as far from a regular army as one can get, and still they definitely are a bane of the Galaxy.
Reasons for becoming a pirate are as numerous as pirates themselves. Some are exiles, some want to prove something, some are led by simple greed. Different gangs may have different motives, but they are still what they are: groups of criminals, ready to gather under a strong leader and try their luck.
However, facing determined opposition, they quickly lose will to fight.
Captains prefer to use crews and supplies that they can rely on, so in most cases captains make their own kind officers and use familiar wargear.