There actually is a tremendous amount of DE FACTO "dead time" in soccer - look at how little time many of the players actually spend running at full speed.
If your job was to run up and down a field that's 100 by 130 yards for 90 minutes with only a 15 minute break in the middle you wouldn't go top speed either. These people are trying to win a game, not all die of massive coronaries before the final whistle blows.