can'tkeep I think if you can manage to do a degree in what is essentially your passion or hobby you are on to a winner. Sadly I don't know of any degrees on Harry Potter or YouTube (DD's passions), so she is going to do a degree in what she is good at and what she is interested in (rather than passionate about).
"Or because so many students go to university as 'the next step' without any particular burning desire to study anything at all?"
I think you have hit the nail on the head here. In my (not vast) experience all but 2 of DD's friends went to university because it was expected of them, and it was just the next step, just like going to secondary school after primary school. University was heavily pushed at DD's school.
Sadly, I know several graduates with degrees who are out of work or working in jobs with minimum wages because their degrees are in highly competitive subjects, subjects that don't offer transferrable skills or subjects that aren't required in the workplace, so please forgive me if I am more pragmatic about why many students do degrees in subjects that are more likely to get them a job.
We live in an area with a high unemployment rate, which probably has influenced the decision making process for our young people.