This. The mainframe model seems bizarre to people experienced in modern programming, because they can't imagine all the things that were different.
So imagine you're a company selling low volumes (by modern standards) of exquisite computing hardware to highly demanding customers, in a world where... (1) there is no Internet, (2) there are no personal computers, (3) most people don't know how to use a computer, much less program one.
You'd probably come up with a model that looked a lot like "We'll sell you the machine + train you how to program and use it!" as a package.
And then after the world changed, but your offering was very polished, you probably just wouldn't overhaul it. Because it's been working fine for 50 years.