I think it's because it makes it easier to rig their arms (although usually it results in odd shoulder creases, etc)... I also think it could be an attempt at making it easier to create clothing for them... plus (for people using the box modeling method) it's easier to model them straight out instead of bent...

BUT... of course not everyone is gonna do it this way, there are many modelers (past and present) that model their figures with their arms halfway down, of course I'm pretty sure that it would be impossible to make clothes for a figure with their arms all the way down.

I, myself, am making a human figure with the arms halfway down, mainly cause it looks much better in Zbrush and (if I rig it right) will have better shoulders in Poser, lol.

 

EDIT: Posted at the same time as Sam so I agree with Sam, but I tried the arm lifting thing and I had no problems bending my arm (completely straight) at the shoulder above my head... I did it with my arms out like Frankenstien and also did it in the T-Pose and I'm very confused by what you (Sam) meant, lol. XD