Sure, but that's not the same every day. I don't want to shuffle my music collection around constantly; I'd rather just have it, so if on a trip I'm reminded of $bandname and want to listen to an album, I don't have to futz with downloading it on hotel wifi. To me the cognitive overhead of manually operating some kind of caching scheme, where I purge rarely used files to the archive and then have to page-fault them back in again, is not worth it.
It's simply the philosophy that most of the stuff we keep with us on our TB hard drive laptops isn't actually stuff we need to keep with us.
Obviously Apple considers your philosophy to be an edge case to their intended business model and would rather not offer you moderately priced options to upgrade, which is certainly a shame.
Whilst I love the 10" form factor and the light weight of the Surface there are times where I need something obscure and rarely accessed, and don't have my drive with me. This is frustrating beyond description.
Also not sure about the data loss argument, as I back up regularly.
There's many options for providing yourself access to the desktop at home from a mobile location, at which point you can access those obscure files.
It does seem that the tablet isn't a great product vehicle at the moment for a Tb portable desktop.
They're responding to a different philosophy.
Take music: The main case is generally "I want to listen to music that I like" or "I want to listen to (specific) music from (artist)" of some sort. I have 150GB of music, but neither of those cases require most of it as most of it is only accessed very rarely. The music file storage can stay somewhere else, for occasional reference if I want to pull something esoteric from the collection. "Cloud" or an external drive are great for this.
Wash, rinse, repeat for photos, movies, old software, etc.
And when it "breaks", i.e. that single-point-of-failure HDD fails, as they do often catastrophically without warning, the files aren't lost. It's like keeping your will, the deed to your house, baby photos, etc. in the briefcase/backpack you carry around all day. Fine way to protect them until something happens to that briefcase.
It's liberating dropping 3 pounds from your shoulder, as well.