Because of sprawl

Hills exist, sometimes you need to go 20 miles, we have record setting temperatures every day, public transit sucks most places, and a lot of areas don't have bike lanes so you keep up with traffic or get ran over. E-bikes are good for everybody.

In addition to RiderExMachina's point, an e-bike will get you to your destination quicker, and with less effort. With how hot things are getting, it's much more preferable to not arrive sweaty as hell due to how much work it can take, plus it's better to spend less time in the heat.

Why do perfectly healthy adults need cars? Or regular bikes? Has everyone forgotten how to use their legs?

I think you're being needlessly judgy, ebikes are great and it's never been about whether you "need" one or not.

They're faster than regular bikes, allowing you to cover a larger distance in the same amount of time, especially if you're fit. They're very fun to ride in general, and they can take some of the misery out of climbing hills or otherwise challenging/tedious parts of your commute. Cargo ebikes can carry a decent amount of stuff and even one or two small passengers in some cases, and in other cases they can replace your need for a car (like quickly getting to a store for something small). And they give you the ability to balance exercise vs convenience as the situation or your personalty demands (you get to decide how much work your body does and how much the motor does).

Finally, ebikes open the door for people to get into using active transportation instead of cars for people who normally wouldn't want to, whether they need help because of fitness, want help because of living in a hilly environment, or because they just want to get from point A to B in a reasonable amount of time. Riding my ebike in an urban environment, I find that I can actually get around just as fast as in a car, if not faster due to traffic.

Because I'm not super fit and live in a very unforgiving and hilly American neighborhood (where I also have to ride on curvy roads where people drive too damn fast) I would have never ever considered getting a regular bike. I'm now riding a bike somewhat regularly, getting a bit of exercise (or not, depending on the circumstances), and having a great time riding on roads, bike lanes, sidewalks, through parks (at a reasonable speed for pedestrians), etc.

Yes, they're more expensive than a regular bike.
Yes, they're heavier than a regular bike.
Yes, having to worry about battery charge is inconvenient.
Yes, it can be dangerous to ride any bike at >20mph.

But as the old saying goes "don't knock it until you try it". Even if you think you're a cycling purist, I recommend at least giving ebikes a try before judging others for using them. I think if you did you'd find that ebikes are an ally of and complement to normal bikes, and just like an electric guitar amplifies your strings, ebikes are amplifying your legs and not rendering them obsolete.

I am long since past my teen years, but as an avid traditional cyclist who is now an ebike enthusiast, here are a few points off the top of my head:

• ebikes are consistently faster. It is easier to plan your day around ebike trips, since they take about the same amount of time every day. With a regular bike, your trip could be twice as long today because yesterday's tail wind has been replaced by a stiff head wind.
• Issues involving extreme heat and poor air quality (in my experience, these often go hand-in-hand) have less impact on ebiking.
• Terrain not being an impediment gives you more options. There may be some path you'd never have contemplated before since it is hilly or goes down into a deep ravine you will eventually have to slog your way out of, and so you'd wind up taking busy city streets instead with the danger that entails.
• ebikes do give you exercise. You can usually control the amount of pedal assist or even turn it off for a real workout. When off, you will get more exercise than with a traditional bike since ebikes are heavier. But you can do this exercise wherever it is safest to do so and go electric when you need to move with traffic.
• If your city has a main corridor for cycling in terms of say an off-road paved trail to downtown, but you'd have to go out of your way to an extent to reach it, you will be more likely to do so on an ebike. It is just not as much of bother to seek out the better and safer routes.

Whatever it takes to get fewer people on cars

With my e-bike I'm as fast as cars in city traffic, which means they don’t attempt to do horribly dangerous overtakes, keeping me alive for longer.