>>274142465
because you need to detach your firing hand. Everyone who hasn't handled guns in real life seems to use their leading hand to do things, which is wrong, you use your trailing hand.
Hold the foregrip to rack, let go of your trigger hand. 
Sequence is like this: mag release button (if present), release foregrip (on most) pull magazine, insert new magazine, hold foregrip, release trigger hand, rack weapon, return fire.

Also I cringe seeing videos of you guys releasing the foregrip to use a right sided bolt action. WTH. Do you like losing your target as you fumble with the gun? think about it like real life and it works much better. 

https://www.youtube.com/watch?v=5ETfT7OMFXM