You're missing the fact that most courses (i.e. the ones that aren't wildly successful) have a large upfront cost. The marginal cost might be a tiny fraction of the price, but that doesn't necessarily translate to a profitable enterprise overall.
You can see the same effect with railways and aeroplanes. There's definitely plenty of operational costs, but the capital costs are relatively high so you want to keep your capital goods (the railways/trains/planes) active all the time even if you only make a tiny profit on each service. However, these businesses often only pan out if they can attract high-margin customers (e.g. business class flyers).
You can see the same effect with railways and aeroplanes. There's definitely plenty of operational costs, but the capital costs are relatively high so you want to keep your capital goods (the railways/trains/planes) active all the time even if you only make a tiny profit on each service. However, these businesses often only pan out if they can attract high-margin customers (e.g. business class flyers).