That blows my mind, I would expect maybe 1 or 2 passengers per plane at most. I'm trying to think of what factors would cause that many no-shows, it has to be mostly missed connections?
I can't imagine spending hundreds of dollars and just not showing up.
A friend of mine works for a Management Consultancy firm and they have full flex tickets if they miss the 8pm flight home they can take the next one or fly back the next morning. All without additional fees. So I believe business travel is the biggest factor when it comes to missed flights.
Side note: His employer is the biggest client of a major European airline.
Keep in mind they sell a lot of tickets where one of the features that allows for a premium price is that they allow late cancellations or changes to other flights. Holiday travelers are pretty "reliable", but business travelers might have changed needs at the drop of a hat (say you meet another prospective client on a business trip and decide to stay another day to fit in a face-to-face meeting).
I'm sure other factors such as sudden illness and migrateable tickets make a sizeable chunk too.
Remember lots of business travelers have connections or flex schedules. When I had to go to the West coast for business, I usually have full fare tickets and book a later flight. If I had flexibility, I’d switch to an earlier plane or first class.
European airports in 2019: A lot of these would be 10 euro Ryanair/Easyjet/Whatever flights, probably.
(The really ultra cheap Ryanair flights are less of a thing now, but in 2019 they were very much a factor)
Delays getting to the airport and missing the plane. Cancelations with full refund. "Hidden city" ticketing. Layover delays. Businesses booking blocks. Flexible flights ( https://www.travelperk.com/guides/flexible-travel/flexible-f... ). Changing / rebooking flights for an earlier or later time.