This page in a nutshell: Wikipedia grows organically, thus the quality of pages is varied.
The skyscrapers in this city are being erected in no particular order or pace
Wikipedia is not being written in an organized fashion. Thousands of volunteer editors work away on millions of articles, depending on their interests and free time. There are no senior editors or managers directing which articles should be created or which articles should be improved. What does this mean?
Wikipedia is incomplete. A lot of information you think would be on Wikipedia may be missing. But that does not mean it will not be added one day. And if you feel you can add it, go ahead!
Many articles, project pages, categories, and templates may require cleanup
Information may not be in the place where you think it would best belong
Some information may be hard to find
Some articles may be much more detailed than others of similar importance. In particular, there is evidence of gender bias in Wikipedia and racial bias in Wikipedia, regarding the fact that there are fewer articles on women and people of colour.
Some articles may not be well categorized. Some categories that could exist may not exist. Some categories may be overloaded, and creation of subcategories may be useful.