You cannot put all the stuff in one namespace; how would you then know what it actually is? For example properties show a list of pages on which they are used (plus the value they have there). This information would need to be represented in another way if it all was in a single namespace. The current approach makes it very obvious what the thing is, a property, type or something else. Also, it avoids naming conflicts. I honestly don't see why you'd put everything in a single ns, it seems it'd only cause issues and not provide any benefits.
Jeroen De Dauw
Don't panic. Don't be evil.