Why Apple is not giving application lock to iPhone?

It is up to the developers of individual applications to provide the ability to password protect them. However, your iPhone will be quite secure and private it you put a passcode on it. All of your data will be encrypted and no one will be able to see it.

It is up to third party developers to write their apps so they lock. All of my banking apps require a password or Touch ID, as does my medical records app and my password keeper.

Never, ever allow someone you don't trust, regardless of their relationship to you, use your phone unsupervised. If you must let family use your phone, store the compromising photographs somewhere else.