1) majority of cs models are xmm looking for a quick buck, not professional models. by professional i mean, deriving majority of their income, sufficient to sustain a living.
2) this would affect the way they would treat their jobs. not to say that all of them don't take it seriously, but it's just a bonus thing, why would they be serious?
so you are not dealing with professionals, there is no incentive to be professional since this is just a stopover hi-bye sort of deal for them. even if you kbkb about them, there will always be people out there willing to pay for girls to smile at them.
now - as to why being serious or not matters. if you are serious, you would probably bother to put some amount of effort to build up a reasonably solid portfolio that showcases some amount of versatility or flair. with that, you can try convincing modelling agencies to provide you with bona fide professional models for test shoots.
with the sort of thread that constitutes 95% of all threads in P&P, just xmm acting cute, prancing around anyhow, no mood, lighting poor, just sharp photos of a girl smiling... the modelling agencies look at your shots, they have no incentive to release their models for a while to do anything photographically related with you.
if you're willing to pay, might as well bother to find a proper, professional modelling agency, hire through them. of course, if all you want is a smiling face, then you will have to live with it, that's life. you must understand what's going on. that's what i feel.