Why is the time control for FIDE chess tournaments what it is? Now that from this answer, I know what the time control is, why is it 90 minute base time for the first 40 moves, an additional 30 minute base time for the rest of the game plus a 30 second increment for each move, and not a 2 hour base time with 30 second increments per move? It doesn't seem to make any sense. If the directors already have time to stay more than 4 hours and 41 minutes after the tournament begins, the maximum amount of time that could go by before the 41st move is completed, why are they rushing them to complete the 40th move a whole hour before then? They would still have the same amount of time to complete the 41st move if it was a 2 hour base time and 30 second increments. Are they specifically trying to rate them on their ability to play with that time control so that they'll know that the ones with a really high rating know how to adapt to playing well with that time control?
We can't really answer what all the reasons for the decisions were, as there are many people involved with the committee that decides on the FIDE time controls and they haven't published their reasoning.

So I just want to point out two things:

One, history. A slightly older time control are for instance 2 hours for the first 40 moves, then 1 hour for the next 20 moves, then 30 minutes for the rest of the game. Before that, games were adjourned after move 60. Earlier time controls were similar but with more time per move. So there is a long tradition of time controls for parts of the game and not for a long time period for the whole game.

The reason is that organizers want to ensure that players have sufficient time left in each period of the game. As players don't know in advance whether their game will last 35 moves or 115, it's hard to decide how much time to use on the first 30 moves. A player who wants to have sufficient time left to play the endgame well has a middle game disadvantage against a player who just burns all his time assuming he doesn't reach that point. We want players to be able to play all parts of the game well so we add the time only if it is needed.

Another thing is to save time in case the game is short. Someone on the losing side of a game is going to use all his time to try to save it, so short games would take very long if all that time was given at the start. Tournaments are already very exhausting.