Why not use aluminium to make iron?

Aluminum and Iron are both elements. You can't use one to make another. Well, not without nuclear chemistry.

Did you mean, why not use aluminum to make steel?

The answer is, in a way, we do. Steel is an alloy of iron and carbon or other elements. Aluminum alloys are widely used, in applications such as aircraft and car engines. However, aluminum alloys have different properties than steel - they tend to be more susceptible to heat and not always have the same tensile strength. So engineers have to keep these differences in mind.