It's simple; there are few-to-no costs for doing so (at worst, getting banned and making a new account), but the emotional benefit of upsetting someone else, while attenuated, still deliver significant satisfaction to those who seek it.

For simple trolls (as opposed to political actors), the dynamic is simple to model; you spend time and some effort to create accounts and say antagonistic things while people respond with abuse (to which the troll feels immune), and then harvest (via screenshots) examples of the saltiest tears for sharing with ones troll peers for lulz in other forums.

People do the exact same thing in real life, but it's more time-consuming and expensive to establish and maintain physical groups, both in economic terms and direct costs (legal or physical sanctions).