sptraveler,
Because the automatics aren't "right" of "wrong"; they just are. They are inanimate objects that do as commanded. Garbage in, garbage out. The crew, for reasons not yet explained, didn't know they were doing the "garbage in" part; their training should have ensured that they added power manually when the throttles didn't move.
And, as has been stated, "fixing" the automatics may well ontroduce others risks.
GF