Because they trip much more easily, and they are sinply not needed all the time. Plugging a motorized refrigerator full of perishable food to a GFCI receptacle is a bad idea.
I used to work on residential construction jobsites and the only power on-site would be a pair of GFCI duplex receptacles in the basement beside the panel. Tripping the actual overload breaker was rare (and usually only happened when multiple people were running off a single extension cord), but the damn GFCI breakers on the receptacles themselves tripped all the time, particularly when the table saw was running at full tilt for 20 minutes straight. They're just too sensitive for certain applications. Now, you can buy combined overload/GFCI circuit breakers for in the panel, but they're ridiculously overpriced so no one uses them, and they're probably just as sensitive.
I used to work on residential construction jobsites and the only power on-site would be a pair of GFCI duplex receptacles in the basement beside the panel. Tripping the actual overload breaker was rare (and usually only happened when multiple people were running off a single extension cord), but the damn GFCI breakers on the receptacles themselves tripped all the time, particularly when the table saw was running at full tilt for 20 minutes straight. They're just too sensitive for certain applications. Now, you can buy combined overload/GFCI circuit breakers for in the panel, but they're ridiculously overpriced so no one uses them, and they're probably just as sensitive.