For aluminium conductor, air and moisture readily form oxides and hydroxides when in contact with the metal on its exposed surfaces—coincidentally happening to be superb {and merciless} insulators.
As is probably evident from the images, in time the oxides also migrate into cable-strand interstices.
Nonreversible tool-compressed terminations and splices are unbeatably durable for many decades of electrical-connection reliability [and survival of the electrical system.]
