Because ND doesn't redshirt. And before someone cries foul, and says that they, in essence, do; let's look at the difference.
QB Joe Smith enrolls at Alabama. He is held out of any game action, in his first season, to preserve a year of eligibility. At the end of the year, the school petitions the NCAA for an additional year of eligibility for the player, to replace the one that he "missed", this year. The NCAA pretty much rubber stamps this application, and the player now, at the end of his freshman academic year, has another 4 years of eligibility. All the kid has to do is remain eligible, and earn a starting spot, and he will be guaranteed to have 4 years on the field.
This is called redshirting.
Now........ QB John Smith enrolls at ND. He is held out of action, because the coaches want him to learn/develop physically before he sees the field. At the end of the season, he has used up a year of eligibility. ND COULD petition for that year back, since he didn't see the field, but they don't. So now the player only has 3 years of eligibility left. He has 4 years to complete those three years, but he only has three years. IF the player graduates in 4 years, AND is accepted to a graduate program at ND, THEN the University will petition to have his freshman football year given back to him, providing him with another year. That's why ND has 5th years, and not Redshirt Seniors............. because, if you want an extra year, then you have to succeed as a student, first.
It's a very important distinction, that says a lot about ND's commitment to educating her athletes.