With that model the company can afford to offer far more content than with a pay-once model. With a pay-once model they only generate enough income to be able to offer a book, and maybe a smattering of supplementary material. Go subscription-based however, revenue increases, so output increases and now they can afford to create and maintain a whole lot more while keeping the price affordable to those who need it during the period that they need it.

It's a similar principle to renting vs buying. If they were to offer all of those materials as a one-off purchase at a price that would allow their business to be sustainable, it would cost more than most are able to afford.

If we go back to one-off purchases, we go back to getting less for life as opposed to a lot for a limited period of time. It's a trade off, and clearly one that most people are willing to make.

People get so angry (OP) about the way things are just because they're unhappy in general and looking for something to blame. Not all companies are fair with their subscription models, but most are. Not every company cares about their customers, but most do. Some companies are run by sociopaths, but most are run by normal, nice people.