Because as Warren has said he's trained by a guy that gets given hand-picked cream of the crop amateurs who already know how to jab and fight generally for the national squad.
He has shown this flaw in many fights, crouching down leaning forward flat footed instead of standing tall jabbing and moving. Here is an example of the povetkin fight where he got hurt next to an example of how he should have fought against that style opponent. This content is protected