Probably lack of vision and consequences.
Bayern are a fucking meat grinder. Promising young players who don’t quite make it are shipped out. They have a pretty ruthless streak when it comes to recruiting, they have competent people at board level and ex-players in spots where they can leverage that celebrity for good effect.
Contrast that to Arsenal. Basically zero consequences for bad behavior and bad play. Scattershot approach to both recruitment and retention. Players in semi retirement on lots of money. Zero medium to long term vision. Play poorly? Who cares, you’ll probably get picked again. Act like a dumbass? Who cares you’ll probably get picked again. Not getting picked? Who cares, you’re on a lot of money, living in London and getting paid to stay in shape.