Let me put this is a very simple way, people like objectives ... if you arent giving players goals they will simply see as there is nothing to do and move on, even if you are obscuring it with drops people like being told to do stuff.
What you say holds true only if you operate on the paradigm that the developer provides content for people to consume, and the lifetime of the product is dependent on support and content updates. There are other paradigms though - how about Second Life, or EVE? How about PnP RPGs, which, if you think about it, have a lot in common with SL and EVE in the sense that both provide a toolset for the players but what actually happens and why it happens is almost entirely driven by the players themselves?
Sandboxes dont work because games must have a purpose
Let me remind you what a sandbox originally used to be:
Ever played in one of those? Remember building shit, and then tearing it apart? Remember using shovels as swords and buckets as helmets? Remember just idling with your feet in the sand? Remember dropping into the sandbox and swimming in it, pretending it's the ocean? These were all games, but with self-defined objectives that never had a clear "win" state, or "end" state. Occasionally you would come up with games that had win states with your friends, but these quickly were over and, after a moment of confusion, you would resume doing stuff that doesn't end. Because why would you want your fun to end?
The sandbox was a set of tools given context, but the objectives were defined only by you and the other players. Even today children love playing in sandboxes (unless the parents keep them under house arrest for fear of paedos and germs). No one ever told you what you should or should not do in the sandbox except for abiding by a few basic rules (don't leave the sandbox, don't piss in the sandbox, don't bring non-sandboxy objects into the sandbox etc)
Lego did the same. So does Minecraft. Games like EVE don't really fall that far away from this. It's not everyone's cup of tea, admittedly. Myself, as I grow older, I prefer games with a clearly observable progression from zero-state to win-state, and when a game does not feature such a progression or does not feature a win state, at some point I find the exercise pointless, futile, unfun and kinda depressing actually, and quit. People spending 1000s of hours building minecraft crap prove that there are way different approaches.