Why can’t you use a bonus action in place of an action?

It’s pretty well established that you can’t use a bonus action (spell or otherwise) as your action on your turn. See here and here for those questions. But I still don’t really understand, why are the rules are set up that way?

The answers in the linked questions reference page 189 in the PHB where it says:

You can take only one bonus action on your turn, so you must choose which bonus action to use when you have more than one available.

Does anyone know why there isn’t a caveat along the lines of “you can only take two bonus actions if you use one in place of your action”?

I wouldn’t consider it productive to question why every rule is set up the way it is. But in this instance a quick search online will turn up numerous people who have a houserule that bonus actions can be used as actions, so apparently the reasoning behind it isn’t clear for a lot of players. Something from an official source would obviously be best, but I’d take a tweet from one of the game designers or even specific game situations where this would be problematic too.