You don't expect what you cannot choose to do... The only thing you can eventually do is looking at the initiative order and "try to imagine how things are going to go". Overwatch doesn't allow me to target the ennemy that's about to fire a missile and it's perfectly fine.
Larian doesn't absolutely have to use every words of the PHB to implement reactions. It could be different and still be valuable tactical option to use in BG3... just as overwatch is a valuable tactical option in Xcom even if the rules doesn't allow you a full control over "when and who".
I'm not comparing overwatch and counterspell at all, I'm just giving exemples. Tactical turn based games doesn't absolutely need a faithfull DnD 5e reaction system to be fun and to allow our characters to react during the ennemies turns. Especially if it means "chopping up" the turn based combats even more.