Because she cannot do that. She has trouble against bigger guards and players with length. She doesn't have the shooting ability to consistently make her shots when open. She is listed at 5-7, and that is a very generous listing.
Plus, with Griner and Dupree, there is less versatility than when Penny Taylor was at the four and Tangela Smith was the default five (because every player could shoot from anywhere on the floor). Dupree has a mid-range shot. Griner is showing a mid-range shot. Neither have three point shots. Now, scoring in the paint will increase tremendously (and defense will be much improved). But I have yet to see Prahalis run a pick and roll consistently, which is what is ideal for Griner.
As for Fowles being much younger, she is 26 (turns 27 this week). She was injured this year and had previous injuries, but for the two seasons before this one, she played in every single game (and she played in 25 games this year).