That's not true, it used to exist in the form of FolderSync, a productized version of Microsoft's SyncToy, which later became part of the Windows Live brand (IIRC as Windows Live Sync.)
Then it went away and never came back. Why? Because Microsoft released OneDrive, a DropBox-like cloud storage system, and the 100% free FolderSync was a competitor to it. Microsoft can make money selling OneDrive, they can't make money selling FolderSync, so it's gone.
Basically, the product you're lamenting doesn't exist used to exist, but no longer does because nobody can make money from it.
I used to use FolderSync to exchange multi-gigabyte video files with my friends while we were doing video editing, and it was an amazing awesome product. It was dead-easy to set up, traversed NAT and firewalls without any troubles, maxed-out whatever internet connection it had access to, and used the LAN connection if possible. Now we'll never see anything like it again.
... anyway, TLDR: the problem doesn't "remain unsolved", it was solved but is now no longer solved.