Was that true when these systems were first made? Many of them are decades old, and the culture of large institutions is usually resistant to change, and that comes from management on down. If the system didn't start with those values in mind, what would have prompted them to change?

It's not that I don't expect the codebase to age or design it with long-term thinking, I just don't expect to be working on it for that long. Someone will replace me, and I'd like to leave them with well-designed, readable code that they will be able to understand.

I'm talking about that as well. The modern era of jumping ship every 2-3 years on average in software is not how its always been. Jobs used to be very long term

It's not always about jumping ship either. In many companies, you will move between areas/needs or get promoted to other work after a couple years. Also, companies will outsource maintenance to lower-cost teams. It seems there is better project lifecycle management, less waterfall development, and fewer cushy open-ended government contracts. But, you're right. The expectation that you'll be able to do the same thing indefinitely for you career is gone.