Simply summed up.
Imagine you are Pique. You're 34 years old, and you've won basically everything there is to win in football. You've won numerous LaLigas, you've won 2 Champions League's, you've won the World Cup, you've won the Euros. You've made a ton of money. You are very famous, and everyone who follows football knows your name.
Your net worth is $50 million+, you have a beautiful wife, you have kids, you have multiple business ventures. You already have a foot out the door of football and have many things to focus on in business, and with your family.
What's driving you to give 100% effort on the football field anymore? You have fame, you have accomplishments, you're filthy rich, what more can football give you? You've done everything you wanted to do in your career and you're 34 years old now. The only thing left is fun. You can't gain anything from football anymore, you've won everything possible, you have nothing to prove to any haters or anyone. You have nothing to fight for.
Now imagine you are Ferran Jutgla.
Last season you got released by a third division club and you're nearly 23 years old. Nobody knows who you are and your Insta account isn't even verified. You've probably made >$100k in your career, so you are not much wealthier than a regular person. You have no accomplishments in football, and if you fail at Barca B you might even retire from football. And you are only 22 years old, single and with no outside interests than football most likely.
You have everything to gain from football and all the time and effort in the world to put into it, because you need it. You're not famous, haven't achieved anything in football, and made a little amount of money. On the football field, you have everything to work for, and you'll bite someone's hand off for the chance to play in the first-division let alone Barca.
So someone like Jutgla, despite not being that able, will put 100% if he gets a chance to put on the Barca shirt, and will listen to everything the coach tells them without hesitation, because they have achieved nothing in their career and are hungry to make something of themselves. He is grateful just to have the opportunity and no doubt has an exemplary attitude, and will follow instructions without question.
Someone like Pique won't care too much. They have already achieved everything there is to achieve and there's nothing left for them in football. He already is running his own businesses and has a family to bring up, why would he care about football where he is one of the most decorated players currently active? He has a foot out the door and no incentive to push himself on the pitch anymore.
That's why Jutgla and players in situations like him will work hard every time they puts the shirt on, and veterans like Pique or to some extent even relatively young players like Coutinho or Umtiti won't.
