It's part of the level-based challenge concept of D&D.
As your character gains levels, they get the skills and abilities that allow them to meet and overcome more difficult challenges.
In general, the DC's for the skill tests that you need to make will be much higher at 10th level than they were at 20th level. It takes higher level characters with higher level skills to meet the challenge represented by these DC's.
If you could buy as many ranks of a skill as you had available skill points, low level rogues would easily bypass any trap they meet, anyone could be a master craftsman with only minor amount of training, any low level wizard could be a master of any field of knowledge.
It would ignore the fact that some things are only learned after a lot of training and experience, that you can only learn a subject so quickly. People don't master a skill overnight, or in a single level.