And how are they supposed to kill Savitar when Barry can’t use his powers properly? Even if Savitar forgot how to use his own speed, he’s still in a gigantic ass suit that’s nearly impervious.
And as KF noted, in the time it would take for him to relearn those powers, a bunch of supervillains would have free reign.
Mick specifically dropped him where Thawne picked him up, which was before his recruitment into the Legends.
It makes slight sense, but it belies a greater issue. The short term explanation is that Savitar without the memories wouldn’t have any reason to go back and give Wally his powers. But of course the issue is, Savitar without his memories would also have no reason to go and kill Iris, which would mean that Barry doesn’t create the Time Remnant Brigade and Savitar isn’t created.
The long story is, they had to give a bullshit reason so Team Flash would actually have a decent reason to give back Barry his memories because otherwise they would just leave him an amnesiac and Savitar would be potentially completely neutralized.