They have more players established playing at the level, and who are able to consistently play more consecutive games, with shorter recovery times, and with less of a drop off with fatigue.
'Quality' is to a large extent physically able to play/cope with the level...not so much how they can kick. All of these players are amazing on the ball if you give them the time. But they don't have the time. It's the pace of the games that makes these differences. It's why a player like Sancho can look amazing at Dortmund (in what is arguably the fourth strongest league in world football) but can't make an impact at Manure (in what is arguably the strongest league in the world).
It all comes down to the balance of pace/time. The speed at which you have to effect actions, movement to get free, movement to get in place....and the impact that perpetual motion has on you during games, and game in, game out, week in, week out.
There's this thing about berating players for 'being tired' and that 'back in my day we played three times a week'. Of course you can....but there will be a drop off of level, apart from the best of players. You can play...but you wont be as good, and you'll get your arse kicked. Look at Salah, versus Rashford. Salah is a machine. In amazing shape. Constantly making the same runs, getting on the ball, a constant threat...game after game. Rashfords top level is close. The mean level not so much.