That's because those teams had Messi-Suarez-Neymar in their primes attack, plus Iniesta in support. Or Messi-Xavi-Iniesta-Henry-Eto'o... Or Ronaldo-Bale-Benzema-Modric-Kroos etc.
We don't have the attack to match Barca's, nor the midfield to match Real Madrid's even if we sign some high potential young guy like Ndombele. TO match Madrid's midfield we'd have to sign Ndombele + Pogba/Saul, and that is never going to happen. To match Barca's attack we'd have to bring in Mbappe or Neymar to play with Ronaldo + Dybala.
Our best bet is having a well-rounded team in all departments with lots of depth. If we could afford it, signing Ndombele +De Ligt + Ramsey this summer, and leaving our attack as is, would be brilliant. Unfortunately, there is no way we can afford that without a major sale.