Why is the speed of light constant and independent from the observer?
It's philosophical in the sense that if Lorentz-invariance is part of the most fundamental laws (the final 'Theory of Everything') rather than being derivable from more fundamental laws, then "why are the laws of physics Lorentz-invariant" is just a subset of the general question "why are the laws of physics what they are", which obviously cannot be answered by physics and therefore is a metaphysical question (for example, one answer might be 'because God chose them that way', another might be 'because all mathematically describable universes exist as Platonic forms, and are perceived as real by any intelligent beings that evolve within them').I wouldn't say it's a philosophical question. Philosophy doesn't seem to have anything to do with it (unless you're going to study the meaning of the word "why" or something like that). The only thing that could answer it is another theory of physics. Of course, if we ever find a theory that answers that question, it would give us a new set of "why?" questions.