A lot of people don't play it that way and whether it's a "hard rule" is ... well let's just say I disagree. It's certainly one way.Wow, I didn't realize that was hard ruled in 5e like that. I have only been a player (not DM) in 5e, and we definitely haven't been playing like that.
Personally, I accept that different players are comfortable with different ways of expressing their character actions. I prefer that a player at least tell me what their PC is saying. Heck, I really enjoy doing the back-and-forth in-world dialog. But I don't expect a player to have an eloquent argument any more than I'd expect the person playing the barbarian to bench press the couch.
There are times when I'll clarify what the PC is saying, other times it's such a minor interaction I don't make a big deal out of it.
Do what makes sense for you and your group.