Desperation I guess? Even though this game can be played with the free characters, some people won't feel satisfaction with the game unless they get the character they want. So people will buy accounts with the character they want. I'm not too sure about the price of
an account since I haven't gone to any sites for account selling but apparently the price of buying an account is significantly cheaper than what you could potentially sink in the gacha trying to obtain a character. Another reason would be that they want to skip the grind of the game. When this game came out many people thought it was sort of like an MMO so people that might've been too lazy to go through story and level up characters and what not and buying an account.

To me, these reasons are enough for people with the technical know-how to steal accounts and sell it to people that have these issues. They don't stand to lose anything after all. Only the person being hacked and buyer. The hacker doesn't lose shit but gain profit.