Tennis is a spectator sport, and fans want to see the players compete, not take breaks. If players were allowed to sit down whenever they wanted, it would slow the game down and make it less exciting to watch.
If one player is more tired than the other, they may be more likely to sit down in order to catch their breath. This would give them an advantage over their opponent.
If players were allowed to sit down whenever they wanted, it would be difficult for the umpire to enforce the rules fairly. For example, it would be difficult to determine how long a player is allowed to sit down for.