Steel is cheap, easy to weld, stiffer (per weight) and heavier (per volume) than aluminium - that is to say steel is great for frames.

Aluminium is perfectly usable, but harder to weld, more expensive, etc. Epoxy does not stick well to aluminium without careful prep.

Aluminium profiles are usually "flat enough". That does not mean they are flat, but for a wood router, relatively less precision is required.