why riley no replace fran builds?

[frosty pollen]

Riley does not attack break (Fran's also decreases attack bar).
She also requires skill ups to function efficiently, and there's a lot of different nat 5s that take prio before Riley.
She'll also endlessly stall pve runs and make you waste a lot of time. Getting that 1 extra successful run isn't worth it if it takes 20 minutes.

Fran also full cleanses a target whereas Riley is -1 debuff on team, also on a longer cooldown.
Riley's immunity is worse as well, and her skill 3 requires her to be hit if you want full value from it otherwise it's technically got a 4 turn cooldown.

Ultimately no pve really requires that much sustain. If you do, you're probably not killing very fast and should improve that instead.