Why is speed of light in vacuum absolute?

While studying special relativity at Khan Academy (a website) they took an example where a person was at the centre of an inertial frame of reference and was at rest and exactly at time equals 0 a friend passes him who was seating in a space ship which was moving in the positive $x$ direction with a velocity equal to $0.5c$ and exactly at time equal to 0 the person and his friend turn on the flashlight in the positive $x$ direction. Now when we plot a graph where $x$-axis stands for distance and $y$-axis stands for time, in his frame of reference and examine the graph we observe that the light from his flash light is slower than that of the light from his friends flash light. According to the observations of the universe this is contradictory as light is observed to move with the same velocity no matter in which frame of reference it is observed. But, my question is that as the velocity of other objects in the universe is negligible in front of light and so do not affect lights velocity but do even the higher velocities will not affect light's relative velocity and if not then why velocity of light is absolute? And why are space and time are not absolute?