[ReaperCub]

Steel has the benefit of fatigue limit. Which means as long as the cyclic stress on steel is under a certain amount it won't fail. Aluminium has a much lower fatigue strength than steel and will always fail given enough cycles.

While rust can be a problem it can be mitigated. Also steel is easier to repair than many other materials (welding).

BTW. Aluminium does suffer from corrosion as well. I used to have racing bike, the wheel nipples (these connect the spokes to the wheel rim) used to corrode to the point where they would fail, which meant I would end up with a buckle. I ended up having both wheel rebuilt with higher quality brass nipples.

Plastics under time also suffers from a different set of issues. Plastics can become brittle. Anyone working on old computers (especially macs) can attest to this.