The rear seats in are contoured, for comfort reasons, in such a way that you need to put things under one side of the seat to level them out properly. And then the seats are so close together that leaving access to the seatbelt sockets for the children in boosters to access. So simplistic looks like this give you a really inaccurate picture of the problem.

I will also go further and say that even in larger vehicles getting 3 seats across any single row is hard. Adding a 3rd row doesn't even necessarily solve all of the problems if there isn't an easy path out if you have two kids needing 5 point harnesses done by the parent. Those seats block the folding exit mechanism that a lot of 3rd row vehicles use to let passengers in the rear row exit. You would have to remove a seat entirely to let that action work.

Most seats on the market are wide because they include cupholders and other extras.

Nope. They're always too wide at the head, and it is due to side impact protection/cushioning. It has nothing to do with "extras" (unless you consider safety above mandatory regulations an "extra"), since the seats are narrower at the feet.

I've been in the buybuy BABY parking lot playing with store car seats, including the Radian, for up to an hour before (which is as fun as you can imagine).

The Radian is specifically designed to go three across, that's essentially their niche, and they accomplish it to a certain degree. The build quality of the Radian is frankly excellent, but the kid was uncomfortable after five minutes let alone if it was a multiple hour car trip (and I can see why, very firm/minimum padding). Plus it is unpleasant to install/uninstall (particularly in rear facing).

I think you've tried to prove one thing, but really proved the opposite. You've listed one car seat at the very bottom (that fits due to poor construction) and one specialty car seat that trades a lot for only 3x across. Which is essentially an admission of the reality: Most car seats don't work 3x across, it is rare, and it is problematic.

Rear seats in even large SUVs have very little wall-to-wall room, you pull your average Britax or Graco car seat off the shelf, it isn't going to work 3x across. A lot of people wind up changing cars, or replacing their 1st and 2nd kid's $200 car seats to spend $600 more on three Radians (because even with one Radian and two non-Radians it doesn't work).