Why are history books accurate?

The correct statement would be "history books try to be accurate." As they sometimes are not.

However, history books are more accurate than novels or someone's theory or idea, etc. because "history books" usually give EVIDENCE (proof). Such as: Times, Dates, Names, Unit Numbers (example: the 2nd Battalion 47th Infantry, 3rd Brigade, 9th Infantry Div.), and locations, etc.

Sometimes history books are not accurate. For example it has been stated (by various men, both military & civilian) that the US Army (or US Marines) never lost a battle during the Vietnam War (just lost the war...and even that's been challenged). But it is KNOWN to some GIs (veterans of those battles) that US Army units "abandoned the field" during the heat of battle (which was the traditional measuring device for determining who won or lost a battle...besides the usual surrender or total annihilation of a foe). Therefore, the statement of "...the US Army never lost a battle in Vietnam is false..." as some of the participants have given names, dates, locations, and a time, plus units involved...readers of said book which contains those events agreed, that the US Army had lost the battle(s) or at worst were not victorious in them. Yet, the army claims a victory. So...some books say yes an some history books say no...they "try" to be accurate.