"In terms of the in-story logic, the cultists are zealots of whatever evil powers they worship and probably aren't very amenable to negotiation. To the bandits you plausibly could say 'stop robbing people or we'll kill you all' but there's no real way to know they won't just keep robbing people later or do it somewhere else. Also if you don't kill them you don't get to take their stuff, and 'stop robbing people and also give us all your stuff' is a harder sell. Also also for frankly kinda silly historical reasons many examples of this genre award experience points—the resource used to advance your character's skills and capabilities—mainly for defeating enemies in combat, so convincing the bandits to go away might not give XP. Well, I guess if the game designers bothered to give you a way to negotiate with the bandits they could also give you XP for doing that successfully, but they usually don't give you a way to negotiate with the bandits, in part because the expectation is that you're going to conform to the RPG adventurer trope of killing all your enemies. And that trope got started because killing all your enemies gives you XP. Also also also a big part of what lets the 'singular or small group of extraordinary heroes defeating vast numbers of enemies' formula to work is ambushing the enemies, taking them on in smaller chunks. So if you wave the flag of parley and let them know you're there and thinking about killing them, they could all gang up on you and kill you. Either while you're trying to negotiate with them or afterwards. If game designers bothered to let you wave the flag of parley or design such complex logic for their bandits, which they don't."
"In terms of why RPGs are designed around killing things and often don't give you a way to negotiate with your enemies instead—indeed, the very point of cultists and bandits is to provide you with enemies you have a compelling narrative reason to kill—I think mostly it's just a lot harder to design a compelling negotiation/conversation system than it is to design a compelling combat system. And I think most players find combat more fun. Partially because it's more viscerally engaging, partially because human tropes of adventure and heroism tend to focus more on defeating enemies in battle than solving a problem diplomatically. And part of the fun of RPGs is getting to put yourself in the shoes of an extraordinary heroic adventurer."
"I can actually think of a few RPGs that offered paths to avoid combat by talking to people, but those are usually non-default and somewhat hidden paths. Maybe you have to have a piece of information from an earlier quest which most players would have missed, or you have to have done a side quest to get a certain character to owe you a favor or hold you in high regard and listen to what you say, or maybe you just need to have put a lot of points into your character's speech skill. But the fundamental premise of these paths is that you do something kinda involved and hidden and non-default and you get to feel clever for doing all those things and finding a way to avoid combat. If the diplomatic solution was the default, you wouldn't get that feeling. And usually the actual gameplay of this kind of thing is not very compelling compared to combat—selecting options in dialogue menus, basically."
"And yeah, the dynamite is because I am nonzero worried about being fucked over and very much do not want that to happen. No one has told me I definitely won't get imprisoned and prevented from killing myself, so until I get that guarantee..."