he doesn't really need that root but... I think there's a technical reason for it to be like that but it's kinda hard to explain... knockback skills seem to be coded as vectors, the trajectory that a missile travel is the direction you'll be knocked back in, that's why when you flash Vayne's condemn to a side you don't fly completely backwards but a little to the side
in Poppy's case she becomes the missile but in Lee sin's case... there's also a missile that travels from lee sin to the target in a straight line and that's why they always fly on one direction... the thing is that the missile isn't invisible :x the missile is his foot, if the enemy were to flash just as Lee sin is casting his ult, you would see his foot stretching like Zac's Q xD by keeping the target in place lee sin can complete his cast animation where his foot connects with the enemy without much hassle
Of course this limitation seems very trivial to keep his ult like that but so far it's the only logical explanation I can think of