Soccer is unique in contrast to other sports. The clock does count down having a set time for how long a period lasts. That means to the extent of maybe 20 minutes for hockey to count down until the next period or 15 minutes for football. With soccer, however, the clock tends to go up instead of down as anything could happen in a soccer game.
The reason for that has to be taken into account is the simple nature that the half way point is not always going to be the same. Normally in a game like football, they can start and stop the clock when the whistles are blown. Same could be said for basketball. In the case of soccer, the referee has the stopwatch and thus has to keep track of time on the field.
There is a chance that there may end up being penalty kicks that would add up the time thus the half does not end. Depending upon how many end up happening can make the difference. The game does not end until the last half has been played. While it may seem different, it is just simply the way the game is played.