It's not exactly like that. For example, university official will track you down (through phone call, through help of local official or your high school official/teacher) if you didn't enroll. This is done to ensure people (especially from rural areas) don't give up their admission due to difficulties (for example, economical ones).

Obviously these doesn't help when some one takes your identity and enrolls on your behalf.

Even if there is rejection letter. It probably wouldn't make a difference to the subject of this article. A fake rejection can be easily fabricated (especially if the recipient doesn't know what a real one should look like.)

Also, the subject knows his/her score. The university has limited discretion to deny admission of people above the score threshold. At the end, it all comes down to low social economic status and corrupted officials working against the subject.