Building anything that rivals Wordpress in terms of features, extensibility and ease of use is a massive undertaking.
Building a CMS is relatively easy.
Building a CMS that does everything WP does is difficult.
Building a CMS that does everything WP does whilst still remaining easy to use is incredibly difficult.
There are plenty of blog alternatives out there - but few that you can turn into virtually any other kind of system in just a few clicks. WP can be turned into a whole host of other 'types' of website and you don't need to know anything particularly technical in order to do it.
Even installing Wordpress itself is just a few clicks (depending on your hosting provider).
WP is terrible in lots of ways, but in the ways that most people care about, it does a reasonable-to-good job.