In one of his interviews, Sir Roger Penrose mentioned that when the Milky Way and the Andromeda galaxies eventually "collide" and merge into one, their super-massive black holes will also eventually merge. He further mentioned that the newly formed super-massive blackhole at the centre of the new galaxy will eventually "hoover out" all matter within the galaxy.
I recall reading that this is generally the case with super-massive black holes at the centers of galaxies: they will eventually "eat out" all matter within their galaxies.
Question: why don't the stars and other objects within the galaxies just keep rotating around these black-holes, like they do now? Why will they eventually be swallowed by the black hole's gravity pull?