I've never agreed with the idea that Ben Kenobi's outfit was just standard Tattooine clothing. I was always under the impression that it was definitely Jedi robes. His costume is VERY much reminiscent of samurai gi (shirt) and hakama (big, puffy pleated pants that look more like a skirt). Then they threw a monk's hooded robe over the top of it all to visually reinforce the "warrior monk" concept. And early concept art supports this idea. The drawings below (especially the ones with the shoulder pads) are almost straight up copies of samurai clothing.

Also, the fact that Darth Vader, a fallen Jedi, also wears similar robes (in addition to his cape and CLEARLY samurai-influenced armor and helmet) lends support to the idea that Jedi wear samurai-inspired robes. Below is a typical samurai out of armor.

The fact that desert dwellers also commonly wear robes and flowing garments (even here in the real world) in my opinion, just comes as a coincidence and helped Obi-wan blend in.

That being said, I have no problem with, and actually like the idea that Luke's black outfit in ROTJ is ALSO Jedi attire. Just like the samurai (or any other warrior class) from actual history, it makes perfect sense that the Jedi would change out of their every day robes and into specialized clothing and/or armor that would be more practical or better suited to combat. Luke's black costume would be a perfect basic outfit for the Jedi Knights on a mission or something. And it's worth noting, even Luke wears a brown hooded monk-like cloak over this outfit in the opening of ROTJ...