Why is it so common to join extremes without watching a guide first?

[LianaThorne]

While I haven't experienced it myself, I've seen enough people talk about it to know that it is common when doing Mentor roulettes.
Often sprouts will join EX duties using Duty finder instead of PF, which comes with a whole range of issues.

Some are not very polite when people attempt to explain the way things are usually done.

I mean, from my memory back then the game doesn't tell you to use party finder when unlocking the first extreme (I believe it's Levi but I might be wrong). So in that case, unless someone goes out of their way to educate said sprout, they'll just use Duty Finder. You can't really blame them for Square's lack of understanding that JP and NA/EU operate differently for harder content (JP does things mainly through DF with the assistance of macros if you weren't aware before). I also don't believe they could even see on the Party Finder that people are queueing through there for other types of hard content since they wouldn't even have it unlocked (unless they got lucky when another Levi EX was being done through there).

As far as people being rude, it's unfortunate but this is the internet where people don't really feel a need to be as closed mouth about what they're thinking. This will be something encountered anywhere. All you can really do is kick them or leave yourself if kicking isn't an option, but their personality probably won't change by just asking them to stop. I mean it's possible I suppose.