It’s the same reason the moon and sun seem to be going slowly—kind of what @RealEyesRealizeRealLies said. Or why when you look out a car window at the scenary, the stuff very very close to you seems to be whizzing by, but the stuff far away seems to be going by slowly.
Think of like a triangle-ish: <
The pointy end is the object. The two lines protruding out from it are the lines of when you seem to first encounter, then pass, it. Notice the open end, farthest away from the point, will take the longest to go line to line. It’ll feel like you’re traveling slower than if you were closer; really you’ve just got farther to go before the object is behind you.
And I’m no physics person, so if that’s not what’s really happening somebody jump in… that’s just how I’ve always thought it worked.