OP: Zetas take time. The Rey squads that popped up at first were likely not fully zeta'ed. The amount of zetas in a Rey squad is at least 7: Three on Rey, 2 on R2, 2 on BB-8 (though I'm guessing everyone only ever does one on BB-8). Plus whatever zetas appear on the other two in the squad.
Not to mention that the other squads are surely trying to constantly improve their team, adding gear 12 (which also takes time), and possibly faster/better mods.
It surprised me not at all that Rey teams have gotten better over the past month that we've had her.
You're also forgetting confirmation bias. Rey squads also have a bit of RNG, and that RNG turning against you for a day or two, even in just a slightly higher than avg rate, will extend our human bias to believe there's something even more out of whack.
Also, as the last month went on, more people such as f2p players have had time to gear their BB-8 and Rey and switch to Rey teams. Teams in the 20 - 50 that you weren't paying attention to before are now able to hit you and drop your team farther.
