Why don't we use indicators in redox-titration?

Because the end point can easily be observed.

Example: consider the titration between potassium manganate (IV) which is purple and ammonium iron (II) sulfate - colourless.

At the end point the colourless standard solution will turn pale pink when the correct volume of deep purple titre is added. No indicator is required. Also this is not necessarily an acid base rxn