Two reasons.

First is age. These devices are 5-7 years old. Many are using their original batteries, which have several dead cells. So they literally have less capacity than they did back then.

Second is JavaScript, in my opinion. I can often see my battery drain in real time when I use the browser, even when Low Power Mode is on. Demanding client side JavaScript is a big reason for that.

You also have extensions doing stuff on pre/post-load that also contribute to power usage.

Finally, so many web developers test against modern devices, and there isn't enough demand to warrant accommodating less powerful CPUs/SOCs. You can try to use the web without JS, but you'll find that many websites will think that you're a bot and block you.