[Google Pixel 5 Questions & Answers] V0latyle:
Eh...It's a compromise. More powerful CPUs demand more power. If we had very low end CPUs in our devices, we'd get incredible battery life with our 4,080mAh battery.

I can't really speak for iPhone, but they all have smaller batteries than we do in the Pixel 5 - even the 12 Pro Max only has 3,687mAh, about 90% of our battery capacity. But...Apple has been using their in-house Bionic SoCs for many generations now, and have had the time to make a lot of adjustments and improvements. Google, on the other hand, was still using off the shelf Qualcomm Snapdragon SoCs for the Pixel 5; while Qualcomm has certainly done a lot to improve their chips' power demands, you have to consider the fact that Apple uses proprietary hardware, where everything in an iPhone is designed specifically for an iPhone, whereas the Pixel devices have typically been a "mash up". This is something Google is trying to improve with the Tensor platform in the Pixel 6 and 7. This is also why the ARM design focuses on multiple cores - several "little" cores that aren't very powerful but don't require a lot of power, a few "mid" cores that have more computational power while still being fairly economical, and one or two "big" cores that provide the best performance at the cost of high power demand. Kernel throttling tries to optimize this by providing a balance between using the little cores as much as possible while still providing good performance; you could use a custom throttle and keep everything on the "little" cores, but your device would be extremely slow and laggy.

Even if CPU development stopped, batteries wouldn't improve much, because you can only pack so much capacity into so much space. Lithium batteries are made of a long sheet of dielectric polymer separator; on one side is the positive "plate", on the other side is the negative. The sheet is rolled or folded in such a way that the positive and negative plates are facing each other, separated by a thin layer of electrolyte. This is where things get tricky - the electrolyte cannot be too thin, otherwise you run the risk of the battery shorting out internally. Lithium polymer batteries also need "room to breathe", or rather expand, as they do during charging; insufficient room for expansion results in the plates being compressed together, which results in a short, and shorted lithium batteries almost always go into thermal runaway and catch fire. This is what happened to the Galaxy Note 7 - Samsung put too large a battery in too small a space, so when the device was charged, the battery didn't have room to expand and shorted out. This is why you should IMMEDIATELY stop using a phone if it seems to be bulging or expanding anywhere - your battery may explode.

I imagine as battery technology improves, we will see higher and higher energy densities (higher capacity batteries for a given volume), but since there haven't been any significant breakthroughs, we are pretty limited by technology. CPUs will continue to improve, but it seems that power demands easily outstrip what contemporary batteries are able to provide. Consider for example the original iPhone - it had a 1,700 mAh battery (41% of the Pixel 5) yet it still managed around 6 hours of web browsing, and if you didn't use it at all, could easily last a week on a charge. But, that was also before we had so many background apps running all the time...