But it's never made sense to me. It doesn't seem like a part that takes a lot of physical abuse that would result in physical cracks (discounting the temperature fluctuations it experiences).
Don't discount those temperature fluctuations, and try to understand the temperature gradients between parts. That exhaust system can glow red-hot at full climb power, so expansion and contraction are a big issue. The riser flange is bolted flat to the head, and the head conducts heat out of it, so that flange is never red-hot or anywhere near it, but the pipe just below will be really hot. The pipe is expanding more than the flange, and that stresses the welded area. As the red-hot pipe is being restrained by the weld to the flange, some compression of the hot metal takes place. When the engine is shut down, that pipe cools, and now it's shrinking to a smaller size than it was before, due to the compressed metal, pulling hard at the weld.
And as I mention, the triangular overlay patches to reinforce that area introduce problems of their own. They are welded around their periphery, but are just snug against the pipe everywhere else, so the pipe gets real hot from the flaming exhaust gases, while the patch gets cooling air off the cylinders. That stresses the patch's weld, pulling at it and often starting cracks. On the 172s I often had less trouble with the unreinforced risers.
EGT can hit 1700°F, right? Steel and its alloys start to glow around 1100°F. So we have intense heat, flame, way hotter than the exhaust system in your car ever sees, and the car's system is made of much thicker stuff. How often does the car's engine run at full power?
Add in vibration, lots of it. And the exhaust itself is made up of plenty of corrosive compounds that eat the system from the inside. Eroded tube ends inside the riser flanges, slip joints and muffler connections are common.
One has to take exhaust system integrity seriously. Beside the risks of carbon monoxide, escaping hot exhaust gases can set stuff afire inside that cowling. Remember that around 50% of the gasoline's thermal energy goes out that exhaust. An awful lot of heat.