What do you think the budgetary cut off is for producing a script with unknown/relatively unknown actors? For instance 'Chronicle' was made for $12 million but didn't have any big names attached. At what point would most studios go "okay, we need an A-lister"?
If the studio is writing the checks for production they're going to try to plug a name into anything and everything. If you get into the murky world of financing partners and other companies like with Chronicle anything goes.
Also keep in mind with Chronicle, the director did a lengthy amount of test shoots to show it could be done, and the writer was a bit of a name at that time. They were into it more for the vex at a budget and story, then cast. Though, that was some great casting.