Even if it's a win-win, they don't want to do it at the cost of pushing out national applicants, which is the main point of what martinuzz quoted.
It's not like universities have an absolute limit on size.
Well, they do in the case of a numerus fixus studies. For example, when I finished highschool, I wanted to become a vet (wanted that since I was five). But back then there was a numerus fixus on that for a maximum of 175 students per year. I got unlucky in the random lottery and didn't get in. So I went for a studies without numerus fixus, biology. Tried again a year later (propedeutic grade biology allows access to second year of veterinarian studies, so I wouldn't lose a year), but sadly I lost the lottery again.
There's more studies that are capped like that, simply because we don't want to train more vets, or dentists than are needed in society. Graduated academic veterinarians that have to work in uneducated jobs because there are no more vets needed anywhere is a bloody waste of skill and expensive education.
Excessive foreign influx can disrupt that system, because the numerus fixus is based on national job market, not global job market.
There's no research into the effects much yet, because the trend is pretty new, which is why I said there's no proof it actually does, but it's just rational though that will tell you that if (let's stay with the vet example) there's 175 student places per year, based on the national need of new vets per year, and more than 50% of those places are taken by foreign students, that return to their home countries to become a vet there after graduating, we'll end up with a severe shortage of veterinarians in the Netherlands.
Personally, I disagree with banning foreign students to solve the problem. Expanding capacity, and/or basing numerus fixus on EU or global job perspective would make more sense.
Then again, expanding a studies to have 150% capacity can't be done in just one or two years. Universities are limited by the amount of professors and other staff, and real estate.