If you strip away details, you reach a point where all things are equivalent. But when you do, there is a pile of things that are in the “doesn’t matter” / “doesn’t apply” category.
I’m watching a squirrel scrambling around the grass looking for something interesting to eat.
Squirrel’s desire for something to eat is a measable quantity.
Yet desire is a human concept affixed to either Greek gods or squishy things in the body. Measurements only function given certain assumptions that are Earth based and humans-of-today use (we’ll use other measurement systems in the future probably that may be incompatible).
Is the squirrel looking? is it scrambling? What’s real?
Einstein would have a squirrel’s reference frame and our reference frame in studying the squirrel and within those reference frames, it’s perfectly possible to take measurements.
It’s only when you attempt to take all references frames simultaneously that you get a special case – hence special theory of relativity