It stems from the top and the culture has been created by Wooward and the Glazier's focus on social media/branding and paying huge sums of money to players who have achieved nothing on the basis that they play for Manchester United. We're treating players like they are premier league winners and paying them as if they are the best players in the entire world. This is going to create a culture of apathy and toxicity in regards to drive and ego's.
I would say we have a few players with what I would consider have a winners mentality. Ronaldo is the stand out, whatever you might think about him he hates losing and is a born winner. I'd say Bruno has some of those qualities but not to the level of Ronaldo. McT is the closest we have in terms of mentality and fight to a United team of old after that - and although he might be a limited player in some aspects if we're talking purely character and fight he is up there. Outside of those 2/3 the rest are all apathetic, they don't have the right drive and determination and this stems from a few things which i'll list below (as I see it).
We got rid of pretty much every player in the squad over the last few years that had the right characteristics. Irrespective of perceived player quality we had a team of people who hated to lose - think Rio, Vidic, Rvp, Evra, Rafael, Rooney etc, even the peripheral players had that attitude (Evans, Hernandez, fabio) - The players who came in saw the standard in terms of character of what was expected of a United player and they had it instilled in them (and they were scouted to ensure they could handle such demands).
We phased those players out and replaced them with the likes of Di Maria, Depay, Miki, Pogba, Martial. Only Zlatan was a worthy recruit in terms of drive and he's gone now. So every player that comes in learns from the current squad, the likes of Lingard, Pogba, Martial, Rashford, Maguire, Lindelof, AwB, Dalot etc are now the standard - So the standards have dropped and there's no core/majority of the squad to keep them high to foster that same culture. It's simply gone.
Now, this is further compounded by Ed etc selling the club to players based on how many social media likes they can get and how the club will further their brands and sponsorship. The focus is not on football, it's on how the player will be marketed. If that wasn't bad enough, we then pay these players who have done very little the highest wages in the world. You have the likes of Martial, Rashford - who have won absolutely nothing being paid more than the absolute best attackers in the world. More than Salah, Lewa, Mane, KDB etc.
You're telling these players they are worth this much - for doing absolutely nothing in terms of trophies or performance. We then hand out even larger contracts to keep their value - not based on performance but again on financial reports and marketing. All of these things compounded are what has created this attitude of apathy and lack of accountability and character. It's a beast of the glaziers and Woodward's own making and it's going to be a long hard road to fix.