Because it is complicated.
A lower value will use Swap Space less. A Higher Value will make more use of Swap Space.
Utilizing Swap Space takes resources of its own to do, so increasing that value under a Light RAM load can actually have the opposite of intended effect, slowing your computer. However, increasing swappiness under a heavy RAM load will have the opposite effect of that... Speeding up your computer.
...And the same is true in reverse.
So, in short - it really depends on how you use your computer and what software is placing the highest demands. And speed is not necessarily the goal. Avoiding running out of memory is. Reading from a spinning hard drive can be slower than reading from the solid state RAM, too. The more swappiness, the more you may be reading from Spinning hard drive, unless you have SSD.
Swappiness is considered a Fine Tune thing. Default is set to 60.
It can range from 0 to 100.
Tuning your swap value should be done in small increments and tested thoroughly before changing the value to optimize performance.
But sadly, it is not so simple as Increase for faster speed or decrease for slower speed.
Setting the swappiness value higher means that you will allocate More Swap Space to RAM, therefor providing you the most use of RAM. But this is not necessarily the same thing as speed (Rather, utilizing available memory to avoid Freezes and Crashes) and the default 60 is usually sufficient for most users. Most users are not doing like I Do and running Inkscape, Blender, GIMP and a bunch of other stuff all at the same time. While blasting loud music.