Maybe it's an attempt to evade automated systems that check for spam.

Probably a basic way to evade spam detection. If you start sending the exact same message to 500 people, most chat services will shut that shit down in an instant. But if you send unique messages, it makes you look more like a real person, and the chat system may let it slide.

So that their fixed script isn't so predictable that we can just nuke them by looking for identical conversations.

I would say more likely to get around bot protection.

Could be to match the style of the target, to try and make the conversation feel more natural for them.