There's a reason you don't see this type of water heater in most of the world.
It's not that people die every day from these devices, it's that in general this isn't a great way to heat water if you have alternatives available.
I don't know about you, but I would prefer not to receive a shock, no matter how minor it might be, when I step into the shower...
I can count on zero hands the number of times my shower has given me a jolt, even a minor one.
We mostly see these types of devices in relatively poor countries without the means to install more modern and safer alternatives, such as whole house water heaters.
electric showerheads, by contrast, provide instant heat, and they provide it for as long as you care to shower. what they don't provide is hot water in the sink for washing dishes, unless you take the dishpan to the shower to fill it. and if they're incorrectly installed they can be deadly, but that's a pretty rare problem both with hot-water tanks and with electric showers. i've never talked to anybody who knew anyone killed by an electric shower, and the nature of electrocution is that it's unlikely to injure you without killing you
(tankless hot-water heaters, called calefones, are common here in argentina, and they also solve one of those two problems—the other one. rooftop evacuated-tube hot-water thermosiphon hot-water heaters are also somewhat popular here)
i think that, aside from dishwashing, the main advantage hot-water tanks have over electric showers is that historically natural gas has been much cheaper than electricity. since chinese solar panel producers have driven the cost of solar panels down by a factor of 20, that's not necessarily true anymore—but the intermittency of solar-energy production favors hot-water tanks, if anything, even more strongly
apparently electric showers are common in the uk, which is not a poor country: https://news.ycombinator.com/item?id=41250251 but with much better safety measures than i'm used to seeing in latin american countries
the entire house is on a single anemic 25-amp circuit breaker. at a low-flow 6 liters per minute (100g/s) heating from 15° to 40° (Δ temp = 25 kelvin) at 4.2 joules/°/gram, you need 10.5 kilowatts, which is 44 amps at 240 volts. at 120 volts just forget about it
i have had a super cheap non-tankless electric shower in a couple of places i've lived in argentina, which sucked. you would fill it up with water, plug it in to start heating the water, unplug it half an hour later to stop heating the water (hopefully before it overheated), and then open the valve at the bottom to run the shower. dangerous, inconvenient, and uncomfortable
on the other hand, if you're building an apartment building in brazil or a hotel in costa rica, there's nothing stopping you from including an 80-amp circuit in every bathroom for the electric shower. it's definitely cheaper than a hot-water tank or a tankless heater, and it might be safer too, since it might allow you to avoid running gas to the room
Do these countries not have any limit on the current in a circuit? Over here we're not allowed more than 16A in residential use and that's over 2.5 mm^2 wires. What kind of conductors do they use in Brasil for 80A circuits?
You can only heat water so much for the available wattage and when it was cold the water was so cold that only a thin trickle could be heated to a comfortable showering temperature.
i mean it's the same reason most people in the usa don't have a bidet in their house; it's not because they're too poor to afford one. technology adoption is path-dependent and mostly random, and fear makes people conservative. yes, there are a few people who are familiar with the actual risks and rewards of the different technologies, but they're vastly outnumbered by the pre-galilean minds who are entirely guided by conventional wisdom
most people in the usa can count on zero hands the number of times they've sprayed water on the bathroom ceiling from their bidet, too
It's not debatable these electric showerheads are objectively better, just because some poorer communities around the world use them.
(bidets aren't an alternative to a regular toilet, and the fact that you think they are is a minor indicator of why it's difficult for even widespread, highly beneficial innovations to become universal even after centuries)
not only is it debatable whether or not electric showers are better in that sense, i have in fact debated it; i refer you to https://news.ycombinator.com/item?id=41250642 and eagerly await your opinion
The fact that they are only common in poor and impoverished areas, tells you everything you need to know.
This argument is a lot like asserting instant-coffee is better than fresh brewed coffee because some people have no alternative choice.
humans are not automatically strategic. one reason is that they're subject to many kinds of irrationality; the particular species you are currently exhibiting is known as 'the fallacy of affirming the consequent'
a pretty large fraction would continue using electric showers just because they're familiar with them—but that is at best only a very weak argument. we can do better than that by debating their actual advantages and disadvantages, as i did in the linked comment, rather than mindlessly subscribing to vox populi, vox dei
if given the option, free of charge, to install a bidet in your house, would you take it? evidently not, because you don't know what a bidet is or how to use it. the same could be said of most people installing a computer in their house 50 years ago. or many modern innovations today, like a gfci, corningware, and borosilicate oven dishes, some of which have fallen out of use because consumers didn't understand them
using a cellphone instead of a landline was only common in impoverished areas 30 years ago, and even today, having front-door locks that are hard to pick are still only common in impoverished areas. also, speaking lingala is only common in impoverished areas. that's not because there's anything wrong with lingala
finally, as it happens, my wife prefers instant coffee to fresh-brewed coffee