[Stormcrow]

Because skill levels (and most things in GURPS) work on a bell curve. After a certain point, it's not worth it to spend another 4 points for a 1% improvement.

For instance, suppose you've got IQ 10 and you spend 20 points on an Average difficulty skill to have skill-15. At an effective skill of 15, you've got a 95.4 percent chance to succeed.

Now suppose you spend another 4 points on the skill. Now you have skill-16, which gives you a 98.1% chance to succeed.

After about this point, every 4 character points gives you a very modest improvement to your success rate. What you get, instead, is the ability to soak up penalties to your rolls. Your basic chance to succeed is effectively maxed out, but for each 4 points you can do harder things than before. Those 4 points keep you at the higher end of the bell-curve; they don't make basic success more possible.

And I think there's also an element of "Let's not make this too complicated."