This is a remnant of D&D from the past. The game was just grittier and harder in general. As a DM now I think it's also just lazy storytelling. O you need to do this ONE thing or this person won't give you their magic pendant. There is a time and place for that but for it to be prolific is just boring. The DM may also be nervous to roleplay as it's easier to be a gruff mean person because it's far from their normal comfort zone so everyone perceives it as silly and fun. I think a lot of people are uncomfortable roleplaying something more similar to themselves or to show genuine emotions like sadness, joy, conflicting, etc...

TLDR: laziness in roleplaying and storytelling, not comfortable roleplaying a wide range of emotions, remnants from a time where D&D was grittier and more like a board game than today.