Mercurial does have a staging area, it is just turned off by default (and this is a good thing).

Mercurial has a very good extension system, and ships several useful extension in the default installation. The 'record' extension adds the 'commit individual hunks' functionality of the git staging area, and needs a one line "record=" in your config file to enable it. See: https://www.mercurial-scm.org/wiki/RecordExtension

By having some of these functionalities in extensions, the core interface is kept simpler and easier to learn, but the more powerful features are still there for those who want them.