stoppage time after 90 minutes be handled more fairly?

What determines whether no time is added, or less than there should be? Why just 15 seconds? You’re slowed down for the whole match until the 70th minute, then it gives you a boost—restoring your normal speed, so to speak—and you score a goal, but then there’s no time left to get another one; you were held back the entire game, and then it just ends right at the 90-minute mark. The game could be fairer and not hold one side back so much just to benefit the other.