They would also have to give current draw which isn’t really possible since each end user has different apps and behavior. So you more often get standby time or video playback time which are based on an “ideal” (probably non-bloated) clean OS. That’s more useful to an end user but also subject to marketing fudging the figures.

You can often look up the battery chemistry or use an app to access sensors btw.

At the end of the day battery capacity is only one factor of many in battery/charge life and is generally just marketing in the context of phones.