But even if not full on 9-5 (most aren't), if you've a lecture at 10am a seminar at 1pm and a tutorial at 5pm, you can't work during that day, can you? That's the reality. There may only be 20 "contact" hours per week, but if they're spread randomly throughout each day, and through the week, you don't have a long enough time gap for a job. So, you're basically just down to evening and weekend work, which is when the sports, clubs and societies, are run and which is usually the only decent length of time to do longer bits of study work, i.e. long essays, research in the library, etc for which the odd hour or two spare during the day doesn't really cut it if you need several hours of uninterrupted concentration.
Most working students just work in places like supermarkets, Argos, bars, McDonalds etc., where they can just pick up the odd random shift that they can fit in. A typical Uni student is no use for an employer who wants someone to work every weekday morning, or every weekday afternoon, or to cover every weekday 10-2 lunchtime peak, etc. Also not too much use for an employer like a bar or restaurant who needs say staff for the evening peak to start at 6pm, if they've a lecture or seminar or tutorial some weekdays that starts at 5, meaning they won't clear the Uni until after 6 and then need to travel to flat to get changed, travel to work, etc - they won't be there till after 7 - some employers are fine with that, many won't be.
Also remember the timetable changes every term, So if your timetable gave you every Wednesday off (no contact time) for the first term, which works for the employer, come the second term, you may have no days with no contact time, or your non contact time might be Monday morning and Tuesday afternoon, which may not work for the employer!
You really have to look at the practicalities. You can't just lazily say "only 20 hours contact time so they can work during the week" - You have to look how those hours are spread and then think about how few employers will be very flexible enough to want staff to work randomly, often with no longer term ongoing pattern nor commitment.