My personal headcannon is that the frame shift drive keeps us 'locked' into the current 'reference/instance' and the thrusters can only power an acceleration up to the values shown in game. Like, the FSD generates a hidden mass almost which is why we do not accelerate indefinitely and is why our thrusters continually burn, rather than burning up to 500m/s then shutting off as you do not need constant thrust under newtonian physics.
It's a stretch, but in absentia of any actual lore explanation by Frontier, it's what I use to justify it.