Because in a way it has like 64 cores or however many shader units a 9500GT has, and the kind of mathematical operations required for physics can be calculated very quickly by those, apparently.
Whereas the CPU has only 4 real cores which can be used for more complicated calculations and operations but have a lot of wasted potential for the basic calculations required by physics.
Let me think of some stupid car analogy...
let's say you have four trucks and 64 Minis and you have to get 128 people who really hate one another from point A to B without anyone getting killed but you have to decide whether to use the four trucks or the 64 minis (yes, it's ridiculous but with some fantasy you may get the point), so which cars will get the 128 passengers faster from A to B? The 64 Minis which would have to go from A to B and back just twice(since you could only carry one person per car) or the 4 trucks which would have to go the same distance 32 times despite their potentially higher payload?
As you can see, even if you have to use 32 minis to calculate some polygons and shaders, they will still get the job done faster.
That's how I understand it anyway.
Ok, now without cars, you have a supermarket, you need personnel for the cash registers, you can either hire 32 students who can do basic maths or 4 mathematics professors, who can manage more customers per hour?
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