Millsap main strengths is being a highly versatile PF who can blow by slow, stiff PFs, take his big man assignment out to the 3 point line, and score around the basket.
AT SF you take away his dribble game, he will have less opportunities to get off his jumpshot, he will have less room to operate around the basket, and he is not providing that extra spacing to our offense.
The reason why millsap is so valuable is because he brings that unique skillset to the PF position. At SF you arent getting that All star skill set at the big man position, you are getting a mediocre SF who isnt going to be worth the 15 mill+ that he is going to be paid.
Asking millsap to be SF is similiar to asking a player like draymond green to be a SF. He probably can be decent at SF but arent going to get that full DPOY runner up value that you are going to get from him when you play him at PF/C.