OS X/Darwin follows the design philosophy of it's cousin FreeBSD. The base system is only upgraded when a new OS version is released. Upgrades may or may not be the latest version of a software package. If the tool does it job and does it well then it may not be upgraded. The idea is " the lastest version may not equate to a better version" or "if it ain't broke, then don't fix it". So, the base system on your version of OS X will only be patched when there is a security concern or severe bug.

Homebrew doesn't have any existing support to accept license to use for every application. Users of brew just download and install.

If Apple had to implement a package manager, I think they would ask application developers to accept Apple's own license and policies for distribution etc. which may not be acceptable with them.

Also, wouldn't it make Appstore redundant ?

Because they will have to maintain it. If it already exists why bother?