There is no evidence collapse is physical
Collapse as a linguistic shortcut
A "measurement" is actually a generation of entanglement between a system and environment, the joint wavefunction doesn't collapse. The homework problems ask you to compute the marginal spin-up and spin-down probabilities, which can be computed from the joint wavefunction. It is just easier for the problem sets to phrase their question as "probability of spin up".
Collapse as a numerical tool
Collapse is very useful to simplify a problem since it reduces the size of the system we are using. Invoking collapse assumes that the environment is very large and can "swallow" any entanglement it gets with the system. When is this assumption valid?
You can measure a qubit by letting it control the output of a CNOT gate. The other qubit can be thought of as the "environment". However, you can undo this measurement with a second CNOT gate with the same two qubits. The environment qubit, which would have reported "UP" or "DOWN", gets resorbed. You can't invoke "collapse" for this case.
But what about an LED screen which reports qubits as "UP" or "DOWN"? The screen emits trillions of photons which each contain which-path information. 855
You can't undo this measurement just as you can't undo smashing a watermelon. You can safely invoke collapse without getting a wrong final answer.
Collapse as your metaphysical observation
why hasn't everything blurred into complete uncertainty?
This "obvious-fact" is quite literally a boring down-to-Earth concept. But it is actually metaphysics, not physics! This is because (as far as we know) it is impossible to build a machine that can detect itself being in a superposition. Or a machine that detects parallel universes.
Your quote states that your subjective self-awareness is 100% real, as is your macroscopic reality around you. But science doesn't address what it is like to be something: your subjective self-awareness, no matter how real it is, cannot be seen by outsiders and so is metaphysical.
Science can't provide an answer to metaphysical concepts. We have interpretations of what actually is happening at our deepest levels of reality, but we cannot design an experiment to tell us which interpretation is correct.
Searching for collapse as a physical event
There is no evidence it is, but people have tried.
One idea is that you can't super-position two spacetimes, which would make double-slit experiments fail above the Planck mass. We can't get anywhere near this mass in a double-slit experiment and I am skeptical that gravity would be so different at low energy to other forces.
Another idea is that collapse occasionally happens to wavefunctions at all times. The repeated localizations in position space will randomly perturb the momentum and so generate heat energy out of nowhere. This also seems very unlikely. 781