Think a lot of folks have this impression, and I'd argue it comes down to perceptions of authenticity and transparent intent.

The image you describe can feel very forced, and people pick up on that. The question becomes "Why are you putting on a performance? What's your angle?". The person who casually curses or has a couple bumps in their personality feels more authentic - they don't seem to be editing themselves or projecting a carefully crafted image, so what you're seeing must be what you've got.

Isn't to say people can't be genuinely polite, not curse, and just happen to be a bit stiff, or present an image that cursing and being strategically bumpy supports to help hide true intent. But the first one can seem a bit less trustworthy (I think of people recruiting others into cults or providing cover for cult outreach activities when I think about your example. 12 Tribes/Yellow Deli shit, basically.)