I think part of the problem is that the directors in Spore actually accept sloppy work.
If you notice, most of the HK actresses, when they started off were not very good either. The difference is, within a few series, there's usually a mark improvement in acting skills.
It's like, if you are in the company of good actors, and competition is great enough, you naturally have to step up your act fast (unless you have backing).
In spore's case, since the majority are bad actors to begin with, the new ones do not have to try very hard.
. And if you are the kind that could act quite "naturally", you'll stand out like a sore thumb, since over-acting is the norm. So in time you have to succumb to the pressure and act like the rest.