Because they’re footballers and they’re generally dull/uneducated/uncultured. But tbf you could say the same about spanish (spoken just as much or more than english in the world, though not as universal in terms of being the language of commerce).
Even then, from all signs of it, Santi, Nacho, and Alexis have given a good go at it. But one thing is speaking english and another is speaking it with your mates as you would your native language. If you watch Santi/Nacho/Alexis talk in the little interviews or rapid fire arsenaltv bits you can see that it would be hard to have a kind of flowing banter with them. But that’s normal, almost no one achieves that level in a new language in four years, especially if you have a job/are not a linguist/spy.