They're characters.
In a game.
Who exist only to serve the experience of the players.
But you need the illusion that they are real.
It is not fun to engage with Game Mechanic NPC 721, it is fun to interact with Grodi the Goblin Marauder.
So everything that is done to NPCs by players needs to be supported by the ingame fiction.
Narrative controlling player facing rules that can override ingamr fiction are laying the game mechanics bare. It's like seeing the Boom Mic and the green screen in a fantasy movie. It destroys the experience for a lot of people.
They never had free will. They aren't people. They're tools to tell a story. A DM shouldn't get bent out of shape by a game element let the players made some decisions for those tools because they also aren't the DM's characters.
And it shouldn't matter how the players gets a hand on those tools. In fact, it's worse to set it up so only a chosen few special players get to use those tools. 'It's magic' shouldn't be a plot coupon that let's you experience the game in just a flat out better way than other players who made the mistake of wanting to play the dashing, powerful hero that certain people want to disallow.
Declaring 'immersion' only when it disenfranchises certain players is not cool.
It is always that certain classes have different abilities. Fighters are good at fighting. Rogues good at Sneaking and thieving and Wizards are good at magic.
Every class as different abilities to influence the game in different ways.
And the tool to compell NPCs (and even players) to do something very specific is magic. And in D&D every character can get their hands on magic. Command and Sugegstion can be get by magic initiate feat for example.
And the fighters gets so many feats, if that is an ability you wanna have, you can get that feat.
But one character not having all the tools available is the quintessence of having classes.
D&D is not a classless game or society. Not having one specific tool is not a bad thing.
If you want to play a certain type of fighter, 5e has the subclass for you. Compell people - I think an Eldritch Knight can do that. Not sure about the spell list right now.
A Paladin can do it definitely.
You complaing that a fighter can't mind control NPC and bend reality of the world by normal speaking as something unfair is like somebody complaing that it is unfair that a Wizard is not very good at Meele fighting.
That's by design!