Why does Apple always lie about iPhone battery life?

They say it last certain amount time listening to music or using internet even with all battery settings savings there no way you can use Internet for 9 hours. It dies after like 4 hours.

What they do is some lab tests, various settings will be selected, and they will time the results, and the average is what they provide.
So yea if they claim 9hrs then its likely under ideal conditions - and using THEIR settings it will last 9 hrs
but you found out like everyone else, in the real world your doing more than a single task = less time.

It depends on how long you've had it for. My new one lasts about a day with frequent use, however my old phone could only last about half a day with the same amount of use. It's because the more often you charge your phone, the more the battery life decreases (it has about 500 charges before it begins to drop). If you also use your phone while it is charging the battery life will also decrease.

Battery life always varies based on what you are doing, different things use different amounts of battery. After a year of use for example the battery life will also, but typically several years before they would need replacing and by then most just trade it in and upgrade.