Securely erase/overwrite data in iOS

There is really no need. so long as the device had a passcode, the data on it is encrypted. After a standard erase, it would be impossible to reconstruct the data using pretty much any method you'll come across.

Apple will not offer something like this as it is act of telling people their encryption is not secure enough.

But I do wish the same feature becoz I do not think erasing the content with a button is secure enough, I want nothing is able to being decrypted even someone got my password.

If that's actually a requirement, then the iPhone is not the right product for you. There is no way to do what you want. Not without compromising the security of the device completely.

Because you can't gain access to the file system of the device without jailbreaking it, which effectively flushes all the security of the device down the toilet.