Since the breaker box contains all the other electrical safety equipment it would make sense to put GFCI systems here; and GFCI circuit breakers are available which do this. But GFCIs inside receptacles are much more common. Is it just because they're older, because they are easier to reset, or something else?
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).
Both forms exist. The receptacle version can be installed by any reasonably handy individual. The breaker-box version provides broader coverage but is beyond the skills of most amateurs --- I could do it, but I would not be comfortable doing it.
Also, some of us have old boxes which make simply finding a compatible breaker a challenge; manufacturers of GFCIs are not going to make the effort to support all that outdated equipment.
Different constraints, different solutions.
While functionally the same, it is simpler to replace a receptacle gfci than a breaker. It is a bit scary for some to pull the cover off and work on an open panel. And it is dangerous. It is safer to simply cut the power from the panel, switch it out and be done with it. The receptacle also constrains the coverage area, which is an unknown in the case of panel breaker.