Because doing the same workout with less rest is also an indication of progress. So is covering more ground.
Say an athlete's goal for a 5k is 15:37 (75/400). In the off season, maybe an athlete will do 12x400 with 400 jog (2min rest) in 75- just an easy reminder of pace.
At the beginning of the season, the athlete does 6x800 in 2:30 with a 400 jog + 1 min rest.
As the athlete progresses in fitness, you cut out the extra minute rest. Now you have the athlete doing 6x800 in 2:30 with 400m jog in 1:40-2:00 (which adds up to 3miles of continuous running in 16:40-18:00min). Obviously once the athlete's recovery jogs are under 7min pace, the workout isn't has hard as it could be.
So rather than try to hammer them in 2:25, you extend it to 8x800 in 2:30. Once you can do 8x800 at 5k pace with the 400 jog at a decent training clip (for our example, 1:40 or so) you have 6 miles of continuous running, 4 of which are at 5k pace.
Same goes for mile repeats- maybe at the start of the season, you do 4xmile at 5k goal pace but you take a 2 lap jog rest + a little (5 min total). Then you take away the "plus a little" then you cut it to only a 600 jog rest. Then MAYBE to 400 jog rest.
What's wrong with that? Again, not every workout has to be a ballbuster. There is room (and should even be some kind of emphasis!) for MODERATE workouts.