Reddit有個講法係話想佢地練保人
They're so used to Guma being good on his lonesome, there's this subconscious thinking that "Guma would be fine" even though they should be protecting him, thus leading Guma to be caught alone and the team lacking damage.
By removing Guma and forcing the team to play "protect the AD", they hope to alleviate or even removing that thinking altogether, while Guma needs to learn to be more selfish.