It's lazy producing.
It would be one thing if this was the 1960s, and they had to do copy/paste because they couldn't afford to do better, but the current art department could have done a proper, mixed fleet in an afternoon.
Have we been given an official reason on why they did this? Was it a last minute decision to have the fleet show up and that's all they had time for as COVID was starting? And while perhaps lazy, I don't really see how it impacts the show to be honest. At the end of the day it's a VFX shot and those were the ships that showed up in universe. It literally has no real impact on the plot. Sure, seeing mixed classes of ships would have looked better but the story would have still played out the exact same way.