I am not part of the Arduino company so I can only guess about the company's policy.
I believe the reason for the success of Arduino is that the company has targeted beginners and to do this simplicity is key.
SD.h provides beginners with adequate functionality so from the Arduino company's point of view it "gets the job done".
This appears to be an excellent business policy but can be frustrating for more advanced users.
There is plenty of third party software but don't expect the Arduino group to support advanced users.