Well with the same width rim a smaller positive offset would actually be less stress on the wheel bearing. But that would take the barrel of the wheel and move it deeper into the wheel well and look awful and cause clearance issues. The reason you see less positive offset on wider rims is so that the inside of the barrel stays about where it is and all the added width is away from the car towards the fenders. People get confused with backspacing and offset all the time. Good excuse to use MS paint.
Here you can see that to keep the same backspacing when going to a wider rim you need a less positive offset. In FWD/AWD land you never see negative offsets but you do in RWD cars. That's why a nice wide rim on a RWD usually has a nice deep "lip". If you were to keep the offset the same on a wider rim the backspacing would be too high like stated above.
Any of the above combos that move the outside of the rim barrel away from the hub increase leverage that the road has on the hub and increases wear on the bearing. For some reason adapters and spacers seem to amplify this effect more than just having a super wide low offset rim.