Because Form Shift and Anatman work in tandem with each other. Take Eden Prime for example. During phase transition into add phase, I can get SSS and Tornado Kick in before the boss disappears and there's enough time in between to get into position for adds to spawn that Anatman will build up 4 GL stacks again. Not doing so would mean that I would lose out on a huge chuck of damage from Tornado Kick on the boss as I Form Shift myself over and over again just to maintain something that Anatman would build back up in time for the adds anyway. Then immediately after Eden uses her Ultimate, I can Anatman to regain 2 stacks of GL before she becomes targettable to attack at all since I can't form shift to maintain GL during the long animation of that attack and have to build it up anyway.
Anatman has a very valid use in that fight because of that and form shift cannot replace that.