Allah swears (even though He never fails His promise), because in times of hardship, sometimes it becomes difficult to actually believe anything could be better. Allah swears to convince us that He can still increase us in His bounty. If we can trust His promise, then that will give us hope. And that hope will rekindle the flame of gratitude.
Also, He asks for a miniscule of gratitude because when times are tough, its not always possible to look at the blessings as abundantly as they have been showered upon us. So if we try even a little bit, that is enough for Allah to increase us.
It happened to me, and is still happening.