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

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.