It’s usually new lesions, not growth of existing ones. In fact, the old ones can heal up and fade (but not always perfectly, so you still get some residual problems), to be replaced by new ones in different places.
Over time, the body seems to find it harder to keep up with repairs. I always think of it as similar to the old anecdote about repainting the Forth Bridge - it was never finished, because by the time it would have been, another bit needed redoing. I think advances in paint technology have meant that’s no longer true today - what a shame we don’t have a similar breakthrough for MS.