I don't understand why it seems that things happen.

Which things?

Relativity requires that all events (points in spacetime) must be fixed and immutable.

That an event is fixed can been seen through what it means to be an event. I.e. consider the event defined as

E = “pebble hits Rock at R = (x, y, z) at T = T”

The position 4-vector has components r = (ct, R).

Question: What would it mean for event E to not be fixed (aka immutable)?

Answer: Such a thing is meaningless.

But, if every event 'is', then why does it seem they happen? Why does our perspective seem to change?

The existence of an event is a result of the laws of nature and causality, i.e. what nature does with initial conditions.

For example; Event A exists as a result of Newton’s laws and initial conditions.

It seems clear that we MUST exist in some sort of multiverse, if we assume both relativity and quantum mechanics are correct.

You’re attempting to read way to much into events than what is really there.

Do you understand that spacetime is not a product of special relativity? It exists in non-relativistic Newtonian dynamics? All spacetime is, is the collection of places and locations. I.e. the set of all pairs of times and places r = (ct, x, y, z) = (ct, R) where R = (x, y, z), is given the name “spacetime.”

Consider an inertial frame S. Then in S the motion of a particle is defined by y = vt + k. Then this particle moves in a straight line with constant speed v. What Newton called a trajectory in 3D “space” is what Minkowski called a “worldline” in spacetime. In this case the spacetime is 2D, one coordinate for time and one for space.

Nothing about this hints at a multiverse.