Why is Aluminium not welded?

Yes, but some families of alloys are not weldable: 2xxx and 7xxx due to their high sensitivity to cracking.
Moreover, this light metal must be welded with care so that the parts do not deform.
Friction stir welding (FSW) is a breakthrough technology that can be used to join all types of aluminium alloys, including those that are considered non-weldable, such as certain grades of 2000, 6000 and 7000 series aluminium.