You want adults to be very disciplined and rational. That sounds great but are most of the adults in your own life experience at this level? They certainly aren't for me (I'm not sure I am at this level).
And it's not even about discipline; it's lying. If you're not going to do the work, just don't do the work. Don't turn it in. Pretty much every syllabus ever says that turning in work you did not do is grounds for failing the class and possibly expulsion.
And why would we want to pass these people through? Then when they come interview with someone like me, I'm still going to find out within like a minute of talking to them that they didn't actually learn anything, and they won't get hired. Do I want to work with somebody incompetent? No. Do I want to work with somebody dishonest? Definitely no. Those both make my job more difficult to have around. So then they end up with a bunch of debt complaining about how they don't have a job.