Because of 2 Timothy 3:16, of course. "All Scripture is God-breathed and is useful for teaching, rebuking, correcting and training in righteousness".
I know most Christians simply use this as justification for picking and choosing scriptures out of the Bible that they want to accept, while ignoring others that "do not apply anymore" or that have "gone away", but 2 Timothy 3:16 makes no distinction between these 'outdated' scriptures and the ones that still apply. Note that it does not say, "All Scripture..." And before you try to split hairs about the language, what is a law if not something to rebuke and correct bad behavior? Indeed, that's the whole purpose of laws.
Its worth asking, too, what Paul was referring to when he penned these words to Timothy. At that time, the only part of the NT extant was a few letters of Paul. There were no gospels. My guess is that he could only have been referring to the OT.