That is because Lucas thought having Darth Vader's birth and the birth of the twins at the same time on the screen was epic and poetic, and a neat idea.
Anyway they are not true backstories but reverse sequels. How else does Padme die in childbirth, and Leia is Luke's sister and Vader Lukes father. The first case upsetting continuity with Return of the Jedi and the second 2 bringing the retcons of empire and jedi into the timeline before 1977 star wars. In star wars 77 Vader was not the father nor Leia the sister.
Kenobi had 2 apprentices Vader and Luke's father entirely seperate characters. Vader and Anakin would have trained at the same time, Vader Kills his fellow knight and joins a rival school with the powers he learned from Obi Wan. Obi Wan's teachings at first strenghten the sith and not the jedi.
Obi Wan being a liar in a new hope is a retcon when in star wars as originally written he was telling the truth.
Most likely Vader and Skywalker at some point would have that volcano duel where Lukes father is killed and vader crippled and forced to wear a machine man suit.