Flarum's concept is to be slim and offer additional functionality and freedom through extensions. If this freedom and power of choice does not suit your needs, but you're looking for a dreadnought forum application, you'd probably be better off somewhere else.
As for your assumption for sofware writers to write their own text editor, this is simply not true, most integrate an existing editor or another. This does not cost that much time allowing the developers to focus on their own value-added software.
