But from a technical perspective, the math is perfectly capable of describing theoretical universes where the laws of physics change in time or space. For example, where physical constants are smoothly evolving or manifolds that don't look the same in all directions.
The math can tell us what observations we'd expect in those situations, and so far we haven't observed anything to indicate we should relax those assumptions.
If we did observe that the laws of physics were changing, that observation would be considered science in the traditional sense. So it's not that "truth" is shifting, it's that "truth" is a family of equations indexed by some parameter rather than a single equation. That's a similar flavor to the jump from Newtonian physics to relativity.
If you're interested in this topic, a relevant key word is "cosmological principle": https://en.wikipedia.org/wiki/Cosmological_principle
The current models include these assumptions for the same reason they include any other assumptions, because models where these assumptions aren't included don't do any better a job explaining any experimental results. If new experimental data shows these assumptions fail in some cases, then the models can be updated to handle them, same as any other failure has been handled in the past. (Granted each step becomes computationally more complex, so eventually we might hit a limit where it is too complex for a human to understand well enough to operate on.)
There's actually quite a big assumption in here -- namely, that the truth is constant over time and throughout space. (This can possibly be weakened slightly, but that's the gist.)
I personally think the laws of physics probably are unchanging, and I certainly hope they are (because that means the scientific method converges). But whether they actually are is not only unknown, but a question that cannot be answered by empirical science.