From Chapter 1:
Beyond 1st Level
As your character goes on adventures and overcomes challenges, he or she gains experience, represented by experience points. A character who reaches a specified experience point total advances in capability. This advancement is called gaining a level.
When your character gains a level, his or her class often grants additional features, as detailed in the class description. Some of these features allow you to increase your ability scores, either increasing two scores by 1 each or increasing one score by 2. You can’t increase an ability score above 20.
And from Chapter 7:
Ability Scores and Modifiers
Each of a creature’s abilities has a score, a number that defines the magnitude of that ability. An ability score is not just a measure of innate capabilities, but also encompasses a creature’s training and competence in activities related to that ability.
A score of 10 or 11 is the normal human average, but adventurers and many monsters are a cut above average in most abilities. A score of 18 is the highest that a person usually reaches. Adventurers can have scores as high as 20, and monsters and divine beings can have scores as high as 30.
Well, those are really good points. The first one, though, could be taken to specifically refer to the ability score increases you gain as you level up, since that's the section it appears in, and not to apply to other ability score bonuses, such as racial modifiers. The second one can clearly be overridden by exceptions such as magic items and class special abilities, and one could argue that reincarnate is such an exception. (I'm not saying that's what the designers intended or even that it's the best interpretation of those rules, but it certainly makes reincarnate simpler while remaining logically coherent.)