Because recasting Gal gadot, Jason Momoa and Zachary Levi would be a huge gamble at that point. There is a reason Downey Jr. was iron man for all the story of this character : audience were invested in him, and while some people didn't liek WW/84 (I find it way better than the original bt the way), Gadot IS Diana for most peoples, and his Arthur seen through the lenses of Momoa. So it'd be the two most successful IP they have currently in movies being rebooted, which hasn't happened to the MCU until what, twelve years after it's start ?
Would people even want to see a new origin story of WW and Aquaman ? Because it'd be needed then.
No, honestly, if they still want a shared universe (which was apparent with the idiotic idea of lumping the JSA in a Black Adam movie) it's better to keep what worked and was loved by the public.