Because the AI is given advantage to beat players. I've gone 11 mirrors in a row going second recently, I'm on another streak of 5 or 6 at this point. Both in GAC and fleet arena.
It's not a priority for CG because it just means they pay out less crystals to players. Their priority right now is getting their subscription model setup to run so they have a consistent revenue stream running every month. I'm sure we'll get new ships soon enough that will destroy anything we currently have. And if you don't have them, oh well!
Same with dozens of other bugs that impact players, not a priority because it's pvp relevant, not PVE. (Same as why GAC and TW are ignored by CG.)