it is because there is a 2 to 3 month break between the end of one season and the start of a new one. If one day in the game is the same as one week, then a 4 day injury should reset when the new season begins.
I agree with the 8 day reduction in injures between day 28 and day 1.
Also, condition and morale should reset to 100% too. Are there many players in real life who come into a new season with low morale and low conditioning?