Reactions are SO MUCH more than attacks of opportunity in 5e. They are a core part of the system's balance. Predetermining reactions will not work because they often use limited resources. Predetermining reactions will change the balance of class abilities, spells, etc. in REALLY BAD ways.
Larian staff seems to have an overriding directive to make combat faster. That's fine, but the trade-off in this case is unbalancing. Watch any video of Solasta gameplay and you'll never see anyone spend more than a second responding to a reaction dialog.
Change My Mind...
Explain how spells like Feather Fall, Counter Spell, or Shield could work without unexpectedly burning through all your spell slots? How about class abilities like Warding Flare that can only be used a few times a day? And since you only get one reaction per turn, even taking a simple opportunity attack may or may not be ideal every time you could take the attack.
