In truth it’s all economics. Belts are cheaper to manufacture into the engine than chains. And the belts themselves are cheaper. There are also some technical reasons (see below) but it’s really just cost-based. If chains were cheaper to make and install they’d all be chains.
Personally I far prefer chains. I consider having to have the belt changed as just another opportunity for a screw-up. But as has been pointed out, on V-style engines with dual camshafts on each head the convolutions in the chain can become difficult to cope with. Chains are much heavier than belts, and as they run they want to form a circle due to the physics involved, much like a lariat (sp?) forms a circle for a cowboy. It’s easier to control a belt in a convaluted path simply due to the weight involved. And the lateral stresses on the rotating components around which the device travels are far lower.
Water pumps…it’s simply easier, cheaper, and more space effective to add the water pump to the belt drive, especially on transverse mounted engines where space around the engine is at a premium. In trucks, where longitudinally mounted engines are the norm, the water pump will still typically be a bolted on component driven by a fanbelt.
I agree about the fuel pumps. But, again, it’s a cost of manufacture issue. I’d guess it’s probably cheaper to put the pump with float and tank together as a subassembly and mount it than to mount both seperately to the unibody. If you seperate them you also have to then add a fuel level measuring system as a whole additional subassembly, so it makes sense for the pump and th efloat to be in one unit. Saves an assembly operation. There’s no technical reason that I can see.
In design, everything has a tradeoff. They’re basically trading off ease of repair for lower cost of manufacture…which also keeps the final cost of the vehicle down.