If the rule in DnD was that you got an opportunity attack on every character that moved past you in combat, and then BG3 only allowed you to get one opportunity attack, people would be screaming. They'd be saying how the BG3 system was broken and dumb for only allowing one opportunity attack.
People are comfortable with what they know. It's often a shortcut to actually thinking.
There's nothing bad about the idea of allowing multiple opportunity attacks in one round. It's not even remotely comparable to endless actions. Clearly, only so many opportunity attacks can happen.
Sure, I agree that there is nothing inherently bad with the mechanic "multiple opportunity attacks [or reactions] in one round." What matters is the context of the game it's in. In your hypothetical, that version of D&D would likely be *balanced* (at least designed) for infinite reactions. Enemies' stats and CR would be modified based on the expectations of taking multiple reactions; certain abilities like Counterspell, Shield, and Protection Fighting Style would probably have text making them more restrictive and/or less powerful; etc.
And if you simply placed a limit of 1 reaction per round without changing anything else, that would mess with the balance of the game and some people would be reasonably upset for valid reasons. Which is essentially my (and many others') complaints about the current BG3 reaction system. D&D 5e was designed upon the assumption of 1 reaction that you had full control over when to use. If Larian wants to change that, they need to consider the cascading effects their chosen implementation on game balance, and modify either the reactions or other mechanics to prevent a reaction from being way OP or next-to-worthless.
Your unlimited reaction suggestion *could* work fine, but it'd require a lot of reworking (of reactions, other abilities, or general game mechanics) to recapture the rough balance that reactions currently have in D&D 5e. And even then it's not necessarily better than one of the many other ways of implementing reactions.
