I guess it is because ALL your exemples (in which you always know what's coming next) attempt to prove that you could make a better choices with pos-up You'd probably have to change a few things at every combats, obviously (mostly targets). But don't forget that most of the examples given in the previous pages are all about the wizards and eventually the sorcerer...
Well, here's a different one for you.
Let's say you've given someone Combat Inspiration: AC increase. They're being threatened by a Minotaur who has a weapon that can potentially do 2d6 damage + 1d6 fire damage if the attack lands, and a goblin with a dagger that does a measly 1d4 damage. But the goblin goes first in initiative. A simple toggle system isn't feasibly going to differentiate between the two attacks. In order to automate this in a way that the Combat Inspiration die would be saved for the Minotaur's attack instead, you'd have to design a system that would differentiate between possible incoming damage, and set a condition to not automatically use the Combat Inspiration if an incoming attack will inflict less damage than whatever number you have set.
In any case, a simple pop up system is far easier to program if you want to retain precise player control over how the reaction is used, and something to automate said pop-up should come into existence afterwards.
But we cannot forget that the crux of the matter is that we're all trying to come up with solutions for a problem that is only subjective to begin with. And how much effort does a programmer really want to put into something like this? The Solasta devs evidently cared a lot, but haven't figured out customizable conditional automation (as far as we're aware, I have enough trust in them to believe that they might be working on something like that after they release the remaining classes in a few months from now). But who even knows if it's even a priority or even possible in regards to BG3 at this point?
I had something else in mind that would better stick to what I'm thinking about for a few days : "to have more control over your reactions, you're able to select your targets".
Rather than "giving the character" a on/off toggle on your hotbar, it gives him an activable icon he can also toggle on/off (right click to toggle, left click to activate). When he left click the new available reaction skills on his hotbar, it opens the variation window in which he can select "manual target" or "auto target". => You give your die just as now but rather than only being able to turn your toggles ON/OFF, you can manually select the targets that will trigger them.