Breakers work when you draw too much current through the circuit and a metal strip heats up, expands and flips the circuit off. It's meant to stop you from plugging in too many devices and heating up the wire inside of your wall. It takes a bit of time activate.

GFCI stands for Ground Fault Circuit Interrupt. If any current is detected going across one of the current carrying wires and the grounding wire, it immediately turns the circuit off.

PSA, please, please don't rip off the grounding prong on your outdoor water fountain pump because you can't find a grounded extension cord.

It depends honestly. Here in the states, we GFCI kitchens, bathrooms, laundries, garages, crawl spaces underneath houses, and exteriors, basically anywhere it could be reasonably expected to come in contact with water or an unexpected grounding/earth source. From there, you can either do a GFCI breaker or receptacle. Both will protect everything downstream from the device, but the choice comes down to convenience of operation. I'll generally do GFCI breakers for dishwasher, disposals, refrigerators, etc, just so that if the GFCI trips you can reset it in the panel so you don't have to pull the equipment out to get to it, but I'll do kitchen and bathroom counter convenience plugs as a GFCI receptacle (and daisychain all downstream kitchen receptacles from the GFCI receptacle) to be able to reset it right there at the point of use.

You could in theory GFCI protect an entire house/flat, but it likely comes down to cost saving and avoiding nuisance trips. Motors as they age tend to leak current and trip GFCIs, and any number of delicate electronics can be finicky, so that's typically why we only use them in wet locations.

American homes have a master circuit breaker (or fuse) which protects the entire house, and then individual circuit breakers for several circuits throughout the house, the exact configuration depends on the home, the era it was wired, etc. These only look at current. An American household 15 amp circuit breaker will happily electrocute a human if he only draws 10 amps. It doesn't care if the electricity is completing the circuit through ground, it's only job is to keep the wires in your walls from overheating and starting a fire.

Ground fault circuit interruptors (GFCIs) detect when current is completing the circuit through ground instead of neutral, which is what happens when you drop the proverbial toaster in the bathtub. Fresh water is an insulator but salty, soapy or acidic water is a conductor, and will conduct electricity into the pipes of the house, which are often used as the electrical system's ground. GFCIs are often installed in places where one might encounter the outdoors or plumbing, aka when there's a significant risk of a short to ground. A typical American duplex outlet costs a buck or two, a GFCI outlet costs upwards of $10, it's expensive to install a whole house with GFCIs.

Cheap billion dollar companies

We're Americans, we do things illogically here

In seriousness though, I'm not really sure. I would guess, like most things, money is the answer. The codebook we electricians use specifies what needs to be GFCI. You can always go above that, and make everything GFCI, but you don't have to. If you're bidding a job, you can estimate higher to have GFCI protected everything, but the customer is almost always going to go for the cheaper price, so why bother?

I am an electrician, but this is mostly my speculation and me talking out of my ass so uh... take it with a grain of salt.

Hair dryers have killed people too, which are much more likely to be found in a bathroom. It's why they all have GFCIs in their plugs. I've also heard tell of a girl who was electrocuted in the bath by her phone charger. The USB cord won't do it; the 5V USB puts out can't hurt you. But if the charging brick and 110V wall power cord you've plugged it into falls into the water with you you're gonna do the 15 amp dance.

It really is to save those that aren't smarter than your average Bear from themselves. Logic would dictate that, yes, keep electronics or electrical appliances away from water, but too many people don't connect the dots. Also too, accidents happen to the best of us.