They're really just different types of software that people (gamers) keep treating as the same. The Epic Games launcher was originally only a developer resource for the Unreal Development Kit that has been expanding into other areas after the release of Unreal Engine 4. Going between the two, it's clear that most of their resources still go into the development side.

Valve has focused on providing a great experience for gamers. Epic has focused on providing a great experience for developers. They both do their respective jobs very well, and aren't particularly great at the others'. Valve does far less for developers than Epic does, and Epic does far less for gamers than Valve does.