That rule is not to benefit the lenders, it's to benefit the students. Student loans used to be dischargable through bankruptcy, and when they were (surprise surprise) a significant number of students would go to school, graduate, immediately declare bankruptcy, and then go on with their lives. It would be even worse of a problem now that tuition is so high.
Bold mine for emphasis
I can't find the source so unfortunately you'll have to take this at my word, but this was recently reported to be approximately 1% of all student loans, which is much lower than normal delinquency rates for any type of loan. Also it was discovered that this ~1% was highly concentrated in lawyers and doctors (those who had enormous debts and enough knowledge to beat the system), so they started by making govt loans non-dischargeable for 5 years and from there the lenders lobbies pushed further and further until we arrived today where no student loans govt or private can be discharged.
You are ignoring several complicating factors.
1) You are assuming that the percentage of students that declared bankruptcy in 1976 would be the same as the percentage of students declaring bankruptcy today. Keep in mind that 1976 was far far before the internet and in an age where bankruptcy was far less common, so it makes sense that the only people who would have known of the "bankruptcy option" would have been lawyers. Today it would absolutely be common knowledge, and much greater percentage of graduating students would jump at the chance to discharge their enormous student loans with virtually no negative repercussions.
2) You are ignoring the fact that student loans today account for a much higher amount relative to the student's future earnings. You point out that the students in 1976 who had the highest debts (doctors and lawyers) were the most likely to file for bankruptcy. We are all aware that college expenses have risen much faster than inflation or earnings in the last several decades. It is probable that the increased debt load would prompt many people to skip out of loan repayment if they could, where there 1976 counterparts would have repaid it and not bothered with the hassle of bankruptcy (if they even knew about the option).
At the end of the day if you take away the special treatment of student loans then they become just like any other kind of loan. Mortgages have the house itself as collateral. Title loans the value of the car or whatever. If you want to just get a personal loan you're either going to be paying extremely high interest rates (see credit cards) or you will have to provide some form of collateral that the bank can seize if you default. Student loans would be no different.
It may not be a fantastic situation, but I really think that in order to 1) provide loans to those who need them and 2) keep student loan interest rates as low as they are you really do need to keep student loans not dischargable in bankruptcy (except in cases of undue hardship, as is current law). To do otherwise would only end up hurting the students, not the lenders. The lenders will always do the mathematically optimal thing regardless of what you change the rules to; they'll be fine. The students are the ones who stand to loose big.