I am well and truly old enough to remember 8 hour+ ToHs. I am nowhere near ranking as a "newbie".
And people as a whole, after Ayu, tend to expect custom monsters these days. Not only that, but it keeps things interesting. While you can guess that most people will be snipers, WS or Creators, after so many years, they'd certainly start find it boring fighting the same bunch of mobs that we deem "hard" or "anti" them again and again.
So yes, it's more now than "just approval". We make, we test to see if balanced, we test on various classes with various gears anyway.
We want it to be different, and interesting each time. With 5 mobs per month, you run out of appropriate mobs damn quick.
Even with custom mobs, we need to think of different skill variations to achieve a central desired effect. The same skill set on a different looking mob each month is just as boring.
Nor do I think as much of it is automated as you believe. :X
It DOES still take a lot of backend GM time.