Because we know that was documented, true, in AD 600, that was not uncommon through out the mid east no matter what religion. Heck it was common here in the US/North America to marry at puberty 200 years ago.
Again, the age of Mary has nothing to do with it. Mohammed married a 6 year old. I know of no society where that was common. The evil prophet was a disgusting pedophile, traitor, liar and quite likely posessed by Satan. Other than that, he was an all around great guy I’m sure.