We do have standards and off-the-shelf libraries for everything required to implement this
Yes, exactly. There are libraries available so that you can create your own solution for encrypting files and managing the keys. You can do it, and I can do it, and some other guy can do it, and if anyone is unlucky enough to want to use all of the services we create, then he can have several implementations of what is essentially the same encryption scheme with multiple different methods of managing the many associated keys. Some of the key management will be made transparent by having it automatically managed by software, or maybe it won't. Who knows, because we're all rolling our own solution.
And maybe, just maybe, if we all do things the right way, he can use the same private/public keys for all of the solutions. Except that we don't know what the "right way" is because while there are libraries for the encryption algorithms themselves, there's no cross-platform standard for actually implementing the entire system. Much more likely, he'll be able to use the same keys for 6 out of 10 services if he's a programmer or expert sysadmin, and can recompile of some the open source libraries with the appropriate switches to store data in a specific location... or whatever. It depends. Who knows.
This stuff just isn't going to work until someone actually works out an entire system, and there's a consensus within the community (users and developers on the internet) on the proper implementation. Until then, there will be a hodge-podge of silly solutions that users will be hesitant to use, with good reason.
Yes, I understand that you won't even be able to see the problem I'm indicating.
If you check the research literature then you'll find more interesting schemes.
That's part of the thing, I don't want more "interesting" schemes. I want the internet to agree on one very dull scheme. How to I enable a user, a user who is essentially a moron when it comes to computers, to encrypt all of their data and all of their traffic without any risk of losing data when they lose their private keys. Come up with a single scheme, get Google and Dropbox and Microsoft and everyone else to agree to an implementation that will work the same way across all services. Make it as common as SSL, but make it free. Give me a complete software solution that lets me encrypt my files on Dropbox, Google Drive, OneDrive, let's me verify my identity on SSH connections, as well as sign/encrypt my email. Let me store that key once per machine, securely manage it across machines, be able to revoke it, be able to handle a complete loss of that key. Make this simple enough that I can do it even though I can't configure an IMAP/SMTP mail account. Make the whole thing virtually free.
When you've figured that out, that is when we can have ubiquitous encryption on services like Dropbox. Until then, you're just adding complexity and nightmares to whoever has to manage these things.