Rather than just answering a popup (which totally suck gameplay-wise), you have to think about your reactions during your turns and "cast them".
You manage your action, your bonus action, and eventually depending the situation you also check/manage/re-"cast" your reactions.
Another extremely relevant consideration is turn length. When people have more distinct actions during a turn, their turn length will increase exponentially as they consider all options simultaneously. Not only would players (in the case of pre-actions) have to take time choosing which preaction(s) to activate AND the target(s), they'd also need to consider how those decisions combine with the rest of their turns - action, movement, bonus action. While I don't agree that BG3's decisions should be made solely with multiplayer in mind, pre-actions would make multiplayer significantly more tedious while not really affecting singleplayer turn length (since you're playing all the characters, and thus are theoretically fine however long a character's turn takes). Negative in once case + neutral in the other = net negative.
Whereas pop-up reactions are immediate yes/no decisions, and since you'll likely only have 1-2 reaction opportunities each round (more if you're doing something basic Protection Fighting Style I suppose...), there's many less things to consider. It's basically a single question: "Do I see something that looks likely to happen and would be a significantly better use of my reaction, or should I take this guaranteed reaction right now?"
