I tried to hint at this point, but let me say it explicitly: devices from the same category, like mobile phones of given "class" and year, tend to have more-less the same power consumption. This "tend to" and "more-less" are good enough for rough estimates and comparison. Of course, I would also include other knowledge like common sense (a phone that holds less than 1 day on average is considered broken; no smartphone will hold for more than 2 days in actual constant use).

Also, knowing battery capacity in mAh gives you another funny side effects:

1/ I can ballpark how long it will take to charge it.

2/ I can ballpark how many times I can recharge it with my powerbank (or conversely, what powerbank to buy to have at least two recharges of my phone).

That's the nice thing about such "natural" metrics - they allow for further comparison for different use cases.

This strategy might work semi OK for the same vendor, but I struggle to see how it works for Android vs iPhone since iPhones typically have half the RAM (since it's all native code instead of GC'ed Java) & a drastically more efficient CPU (i.e. runs faster with less battery use at a smaller clock speed than other ARM manufacturers). Moreover, the Apple chips (at least in the past) are able to run all cores at their full clock frequency whereas other vendors have had to throttle down drastically which makes for a pretty large performance/battery life gap that isn't visible from just the GhZ/RAM size/mAh. Even Android vs Android this is difficult to compare as even SoCs from the same ARM family will vary within the same family for the same reason (better thermal design, optimized for GPU performance vs CPU compute, etc). Once you go across ARM families, GhZ numbers are generally not useful for relative comparison & definitely not for power numbers.

I agree you can kind of estimate the charge time via mAh if you pick one charging speed (most now support multiple charging speeds & there's also wireless charging in the mix) but most vendors provide guidelines of charging times you can expect & there's a crapton of benchmarks by independent third parties validating those claims & providing a more complete set of results. Same goes for battery life claims. As for powerbanks, I guess the mAh is useful but there's plenty of info online outside of the vendor's website for that.

If I were the vendor providing this kind of in-depth detail that's demanded by/useful to a very small percentage of the customers would be counterproductive (if I'm providing the info in the best interest of the majority of customers) since it would be more likely to confuse customers who want to compare on just raw specs rather than what the device actually delivers. I can just rely on third parties to report that info for the people who care.

And how will that work with different manufacturers and different use models?

For example I play a certain game. I know how quickly that drains my current phone but I don't know how to compare that to video playing time.

Relative comparisons don't make sense to you?