Why does God torment a human eternally in Hell, despite the brevity of their life in this world?

A disobedient believer may enter Hellfire due to their sins or Allah may pardon them, but they will not abide therein forever.

As for whoever disbelieves in Allah and dies upon their disbelief, they will abide in Hellfire forever, because Allah Almighty called them to His easy command (that they should not associate anything with Him), but they refused, thus deserving the punishment.

As for whoever believes and dies immediately, they will be among the people of Paradise, abiding therein forever, because the good deed of monotheism is not equalled by any other good deed, and the sin of disbelief is not equalled by any other sin.

Allah Almighty does not love praise like Him, and no one is more jealous than Him, and no one loves excuse more than Him. That is why He sent down the Books and sent the Messengers, so that people would have no argument against Him.