In order for any simulation to proceed at a steady state, there must be at least one constant. Everything else can be variable, but in order for time to progress at the same "tick", it has to have something to measure by.

That said, why create a "tick" when the perception of that tick can vary by individual? When I go into code mode, 10 hours can slip by in an instant, and I'm not hungry or thirsty until I come out of it.

Perhaps it's just an affectation of focus, as time still "ticked" by at it's normal rate.

Einstein said that the effects of time move more slowly the closer you get to C, so it makes sense that C would be the univeral "tick" if this was a simulation. As evidenced by Mars rovers and Voyager, an earth clock on another planet or in space still ticks at the same rate (Voyager and Mars are moving through space at the same rate we are moving around the sun, about 61,000km/hour).

There's no other reason for light to move at a steady speed, and nothing apparent that's stopping it from going faster.