Most smart phones are fitted with a minimal size battery due to cost and a market that has placed value on thin or weight over function.
My older HTC One had that battery time issue which was easy to cure with a huge after market battery. I could get days of talk time out of it. But it was over double the thickness and weight after the upgrade.
Most smart phones will never come with long battery times or talk times as the costs prohibit such. My add on battery and case ran over 100 so imagine how many they would sell at the store? Probably none since most folk are going with looks, name or thin. Many are fixated on price as well.