I doubt SE simulates the user actually moving through every centimeter of space, so maybe it 'skips' across at intervals.
Basically this. No computer calculation can be continuous, so your flying through space is not continuous. If you are flying at 10c and SE is running at 60fps, then each frame is separated by about 50,000km. That is several times wider than Earth, so if you were to fly at Earth like this you will almost certainly pass right through it.
At speeds greater than light years per second, you have pretty much zero chance of hitting a planet, or even a star.
I encountered a similar thing in Kerbal Space Program, when putting two spacecraft in exactly the same orbit but in opposite directions, hoping for a high-velocity collision. I calculated the probability of the two craft being inside each other during a single frame to be about 15%, and indeed it took several times of passing straight through each other before a collision finally happened.