Why are aluminium and titanium expensive to extract?

They are reactive metals, and so need to be extracted from their ores using electricity, whereas less reactive metals like iron can be extracted by smelting - roasting with carbon - which is a lot cheaper to do.

Both metals are useful because they both do not corrode, as they both have an oxide layer to protect them from corrosion.

They both need to be extracted by a process called "electrolysis" which means to break up substances using electricity.

Electrolysis uses a lot of energy (ie- very high temperatures) and a great deal of electricity.

Therefore we need to recycle aluminium to conserve energy, as recycling aluminium does not require electrolysis.