In 5e, actions and bonus actions are not exchangeable - you cannot take a bonus action using your action. Different features have a discrete and fixed portion of your action economy which they use, and that is how they work. there are some special cases that allow particular classes to use a feature that would normally take an action as a bonus action, and in those cases the feature does not remove the standard ability to use those features as an action as well, but in general, there's very little crossing between the two. Formally speaking, the main reason for this is system design and balance; for the vast majority of things, bonus actions need to be bonus actions and actions need to be actions, otherwise it leads to instances of extreme imbalance and the negation or obsolescing of other features wholesale. Taking a potion is, formally, meant to be an action; many DMs houserule it as a bonus action for yourself, but still an action to administer to another.
In BG3, the concerns that stand behind the reasoning for this were thrown out the window on day one - Larian don't care about the 5e system, don't particularly like it or have a good understanding of it, don't have any real interest in following it, even where it would make sense to, and they despise the concept of internal system balance with a passion; the more broken and unbalanced something is, the 'funnier' it is to them, and that's a 'good' thing in their game design philosophy.
So, while there are valid and important reasons why you cannot trade down a bonus action into an action in normal 5e D&D, there isn't really any reason why Larian aren't letting you do it in their BG3.
