It's called a "negotiation" for a reason. Agents demand this sort of things in the best interest of the player they're representing. It's no ones fault, just the way thing are.
Btw, this doesn't mean squat. Bartra will leave only if he wants to leave. Buy-out clause only works if the players wants to leave, like Thiago. No one forced Thiago to leave, he chose it. No one will force Bartra to leave, he'll chose it.
Bartra is where he is. He's not better than Pique, he's not even better than Mascherano, and he doesn't bring the same skillset as Mathieu. He played the minutes he needed to play. If he wants to leave to play more, that's his choice, just like if he wants to stay, improve and earn more minutes is also his choice.