Why would they leave the dead bodies of the flight crew in their seats?
If they're dead, removing them immediately is not going to make a difference to them anymore. Ghastly as that may sound.
But their position (who occupied which seat, who was holding which lever, which aircraft part impacted with which body part etc.) may be of tremendous value to the investigation. Also, removing them may mean breaking through structural items and accidentally hitting switches, buttons and such, whose position you might want to record first.
I can perfectly imagine that saving lives comes first in these situations, but once saving lives is no longer possible, preserving the scene of accident is the next priority.