It is, in the same way that light speed is the same for all observers. For example you travelling at the speed of light and turning on a flashlight results in the light emitted from the flash light moving forwards at lightspeed relative to you and we know that you can't go 2x the speed of light which is what would be happening from a 3rd perspective.

So the only way to reconcile this impossibility and maintain causality throughout the system, since nothing can go faster than light relative to any observer, is with time dilation.