Why can't I make an offer different from the rate indicated in the job posting?

Clients exercise authority over the daily rates. They have two avenues: 
1. the option to set a fixed rate - With a fixed rate, freelancers cannot pitch rates lower than the set figure. For example, if $450 is a fixed rate, the offer must be $450, no more, no less
2. Set a competitive range.  With a competitive range, freelancers can craft offers within the range. i.e. $350-$450.  The offer cannot be less than $350 or more than $450.