Phones keep getting faster and more efficient, but battery life has not improved as dramatically as you would expect. Many people still charge every night. The main reason is that while processors and screens improve quickly, battery chemistry advances much more slowly.
Battery energy density
Battery energy density (which is how much power fits inside the same space) improves, but only gradually every few years.
So when a new phone becomes more powerful, the battery inside is not that much stronger. It might hold only a bit more energy than the one from five years ago.
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More features consuming battery
At the same time, phones today do way more than they used to. They have bigger, brighter screens, faster chips, multiple cameras, higher refresh rates, and constant connections for Wi-Fi, 5G, GPS, and Bluetooth.
Even if the processor uses less power, all those features take energy too, so the total drain tends to stay about the same.
The physical design of the phone
Design plays a big part too. Many people seem to prefer phones that are thin, light, and stylish instead of thick and heavy with a giant battery.
So when manufacturers gain a bit of efficiency, they usually make the phone smaller or add new features instead of increasing battery size.
One-day battery life is also just practical
Many people charge their phones every night anyway, so companies design around that routine. Phones that last two or three days do exist, but they are thicker or less sleek, and for most everyday users, one day fits just fine.
Software and app factors
Background updates, location tracking, and push notifications quietly eat power even when the phone looks idle. That is why battery life can vary a lot from person to person.
So in short: phone batteries do not last longer because the technology that stores energy improves slowly, while everything else in the phone keeps getting more demanding. Manufacturers balance power, weight, and looks.
And for many people, one day of use works just fine.
Read the original discussion (ELI5: Why do phones become more and more powerful and efficient, but the battery of most of them still only last one day?).