Because she's BETTER than Shizune.....
thanks for giving me something to laugh about.
rewatch or re read shizune during orochimarus attempt to pursuade tsunade to heal his arms.
then look at how sakura shows that she can fight with sasuke and naruto by "running in beating the juubis "minisplits" and turning her head from battle to show off how much she has grown (from kishi deciding to give her a power up 5sec earlier i might as well add right away) only to instantly need saving or would have been killed off.
yes sakura is stronger than shizune, but shizune wins as long as sakura doesnt land a hit which i doubt she can, unless sakura really rampages and breaks the everything making it so shizune has no stable footing to get away or dodge in time.
meanwhile shizune just has to throw a needle, cause if there is one thing sakura has proved it is that she can't dodge, unless sasuke needs her help.