I did wonder. I was thinking of adding star destroyers arriving at each planet destination before the battle starts to add to the reason why the galaxy got involved but I couldn’t think of a reason why they would do that unless they were xyston class destroyers.
In my headcanon I just assumed the First Order had occupied and laid siege to all those worlds in the aftermath of TLJ, paying off what Rey stated in that movie: “The First Order will control all the major systems within weeks”. Perhaps this detail could be included in a redone opening crawl.