Because they haven't come across the information. They only read one side and do not know the truth.What you see as flaws others do not.
It is not added Text. It was always there. It was copying mistakes which lost words. This is proven by studies in the early Papyrus manuscripts. All manuscripts do it some, but the safeguard is that not all or even most do it, usually. Matthew 3:11 may be an exception to that rule. But if you go by a small half a hand full of manuscripts that lose text it's obvious what has happened.The added text of the byzantine type is considered to be a flaw and thus brings into question the value of such.