The out of universe answer is to allow players to use the colour scheme and name of the chapter while not being prescribed to which army book they have to use. GW also likes to keep players own options open as to the stories they want to tell about the armies they create. So if a player for instance wants to create a loyalist chapter and say they are formed from the remnants of a traitor legion that didn’t turn in the heresy they can.
In universe yes the 40k universe loves its admin, but it is also very inefficient and the nature of a chapter, constantly at war, means that records and information can easily be lost and destroyed. Also the primarchs had gone into legend (pre robute resurection) and the lessons of the heresy meant that various agencies wanted to prevent chapters forming back into legions, so you can imagine that some new chapters are purposly kept in the dark as to there legacy, Gene seed has been taken and used to make them, they don’t need to know who’s it is. Another thing to note is that the Gene Seed changes slightly with each new founding, so a 3rd 4th or 5th generation chapter may actually be, genetically, far enough removed from its founding chapter that it can no longer be called a successor of that original source.
It might also be possible that some of these chapters are made up of a variety of different gene seed liniages, imagine a new chapter being founded in the midst of war from a group of different chapters offering up gene seed, these chapters may come from a variety of sources and so the end result is a chapter made up of 20% salamanders, 60% ultramarine and the rest whitescar.
In the war of the beast series there is real horror that the chapters formed out of the imperial fists have protocols in place to reform as a single force in the event of a real threat to Earth. This demonstrates the very real fear that a series of chapters with the same lineage may converge back to a single legion.
The only founding chapter that is very protective of its legacy is the chapters is the dark angels so you can guess that these legions are probably not formed from them.
For further information
In this article it states that all chapters give 5% of there geneseed to the Mechanicum as a tithe to be used for the creation of future chapters. In this case it is possible that the chapter is never told what there legacy is and which chapter there geneseed originated from