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

[Iyrnthota]

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 solely through DF if you weren't aware before).

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 on the internet. 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.

It is not unexpected for people to turn there first, that is true.

My understanding of the JP EX+ culture is that learning is done through PF, but re-clears/farms are done via DF, so I believe learning through DF would be incorrect in JP as well.

Regardless, I think OP's initial point stands. As broad as the title of this thread is, the content of the OP is actually quite specific as to the context of what irritates them.