Well-integrated solutions were simple until SaaS companies purposefully broke it. There's nothing in software that would make it impossible to have an arbitrary calendar app work well with arbitrary file browser, file syncer, text editor, document editor, etc. A bit of system-level tools (that's kind of what the OS is for), a bit of common protocols. But when each of the above became redone as SaaS, interoperability became a matter of service providers signing agreements with each other, and not something end-users could set up.