Why can't large objects be in two places at once? express
Schrödinger's cat, according to quantum rules, can be both alive and dead at once. But a table can't pull that off. When a massive object tries to be in two places, its mass curves spacetime so drastically that instead of two points, a single deep depression forms, and the object rolls into it. It's like a heavy boulder on a mountain ridge: the faintest breeze, and it falls into a single valley.
For atoms, such curvature is negligible, but for large bodies it's an insurmountable obstacle. Most surprising: the farther apart you try to separate the copies of the object, the deeper the well becomes.
This mechanism is strongest near black holes with their monstrous curvature and somewhat resembles gravitational waves, except here the energy goes into breaking the superposition. Perhaps gravity constantly 'observes' the world, preventing large objects from being in two places — an idea Roger Penrose worked on.
Even a speck of dust weighing a millionth of a gram loses its ability to be in two places faster than you can sneeze.
In Greg Egan's novel 'Quarantine', quantum states control consciousness, but gravity sets a hard limit on such fantasies.