Exhaustion is not only a BG1 and BG2 mechanic, it's part of D&D, and Larian is committed to being as close to 5e D&D as possible. IMO, if you're playing a Role Playing Game, you should expect a level of resource management. Would it make sense if characters were able to fight and fight for days on end without sleep? No.
The longer you go without sleep, the more exhausted you become. At first your skill-checks suffer, then your combat ability. Go too long without sleep and you die. It's not an arbitrary condition. It's meant to provide a sense of verisimilitude (realism).
If you are forcing your players to go without sleep, it is a setup you choose as a GM. And the debuff you get from it should be considered in any encounter you set up after that.
There is absolutely no reason for players to get exhausted in a session other than the GM decides that they will be.
