You have hundreds of dreams a night. You only remember the last one. And since time dilates while dreaming, you really have almost no concept of it at that point, so you come to believe that’s the dream you’ve had all night. But you’ve had countless.
And the reason you don’t remember them all is because they have no impact, like remembering what you had to eat for dinner, 3 weeks ago. Just your brain doing housekeeping.