I would not have given this a thought unless it was pointed out. I'm with you it's not a big deal for me either. It makes sense for Rivian as an "adventure" brand to make their vehicles as simple as possible. Hiding the wiper just makes things more complicated than necessary.I believe Rivian is after the "adventure" buyer, not the "luxury SUV" buyer, so something simple like an exposed wiper on the rear window is probably not a high-priority item. Keep in mind that Rivian is a different type of car builder, so copying what every other manufacturer is doing, goes against that mantra.
As a side note, I think the rear end looks just fine the way it is, but everyone is entitled to their opinion.