I'll tell you why: because the majority of exceptional professional, experienced ones, will not leave their stable jobs in their countries to attend a college program that charges an outrage amount of money for a very ordinary (not to say mediocre) learning experience, while at the same time the government won't allow them to work for more than 20h/week. Therefore, they might burn their hole life retirement savings in that project, to learn the same things they learned in the first year of their University programs, when they were teenagers. I mean, this is what truly happens, based on the testimonials I heard from friends of mine that went study in Canada.
If you have a profile that reaches 470/480+, you hardly will accept these conditions. Only those who can't achieve high scores (because they don't have enough work experience, don't have a second bachelor/or Master's, don't have good IELTS scores, can't speak a third language, etc) will accept these circumstances. The college-to-imigrate path many candidates follow is not what they *truly* wanted to do (most of the time), but what they HAD to do.
Canada might be able to fulfill their gross numbers for immigration only with international students, but this won't grant them the experienced workers they claim they need.
If all Canada needs is people, no matter their professional background, then you're totally right. But as of right now, one of the major claims regarding immigration is that it is needed to fulfill jobs positions that require skilled workers. That's actually the reason why FSW is the biggest category inside the EE entry, both in the amount of candidates, and in the number of PRs granted.