I think it's a culture issue. Which is an organisational issue, like in all organisations.
We are outsiders. And even we can see Spurs isn't about winning.
Now, then imagine how insiders see things:
If an agent has 2 good players, of which one is a genuine gem with a discrete but clear winning streak, where as the other is a flattering talent, who though privately mostly care about leisurely pleasures and money (or just someone who is a brilliant athlete but not nearly robust enough for top level sports). From the outside, for buying clubs, they look much the same - well, if you where that agent which one were you going to push towards Real Madrid and which towards Spurs?
Feel free to disagree, but I will raise you with a truckload of Paulinhos, Ekottos, Lo Celsos, N'Dombeles, Sanchez' and Sessegnons.
I don't disagree. You can't keep getting transfer strategy wrong. But that's where you need a competent team that do their due dilligience.