Because at a lot of fights in endgame, you WILL have a lot of downtime, and it's more than enough to pull up 5 stacks of chakra. For example, Alex Floor 1's boss duo like to fly into the air before crashing at one side of the field. It's long enough for you to lose GL no matter what, meaning long enough for you to have time to fully stock up on Chakra stacks and form shift into Coeurl to get back onto the boss.