Why are GFCI systems typically in the receptacle instead of in the breaker?
The top few possible reasons are:


Ease of access- it's easier to reset a tripped GFCI in the same room.
Easier to retrofit- it's easier for a homeowner to install a receptacle than to dig around in the panel. Even if this type of panel work is trivial, most people just aren't comfortable with it.
Cost- one GFCI receptacle is cheaper than a GFCI breaker. If multiple GFCI receptacles would be needed to properly protect a branch circuit, then this argument would diminish.
Protection requirement(s)- some circuits do not require GFCI protection for the entire circuit, so installing a GFCI receptacle allows you to "target" the required areas (ie, if you don't want your lighting to trip in the case of a ground fault at a receptacle).