Why are only unobserved particles allowed to partake in quantum weirdness events?

Well, let's look at it from the other side ..observed particles don't display quantum weirdness.

Observing a particle after it has hit the final panel isn't an observed particle to nature. The particle has to be allowed to continue on past a detector to be influenced by "observation" in its flight.

Saying "observation" is the same thing as saying "spacetime got involved"

When speaking about "Duality", QFT seems pertinent because it describes spacetime and the quantum field influencing a particle (no quantum weirdness except for wobble ..and the quantum Zeno effect, the quantum field is still making it ageless. )

Decoherence doesn't require a human knowing about it. Spacetime represents our reality and converts virtual quantum information (waves) to physical/real objects.

Observation/Measurement is dead ..it's spacetimes' responsibility.

An unobserved matter wave can have mass and still tunnel ..that means it can hold mass as a variable. It makes sense for Dark Matter to be unobservable matter waves.