I mean to ask if I have a moving object and I turn on a bulb in that moving object, when viewed from an inertial frame, we usually add the speed of the moving object with the object's velocity to get its velocity with respect to the inertial frame.
But as I've heard the speed of light remains same in all frames? Why is that?
Because it doesn't. This fact came as a surprise, forcing physics to revise its models of space and time. In those revised models, the speed of light is the same in all frames, but it is the phenomena that force these models on us. You may sensibly ask "what experiments and observations show that the speed of light is the same in all frames", but there is no "why". It's how the universe works.
