A premade setting doesn't save work, just changes the nature of it.
If you think about it, premade settings don't save the Game Master any work. It just changes that works nature.
It takes a while to read a dense, textbook on a fake world. And it is easy to forget details and need to refer back to it when planning sessions. That is probably just as much time as one would need to make their own world FOR only the details they needed to play that one campaign.
Because a world need not be fully realized. Just finished Spider light by Adrian Tchaikovsky. You pick up quickly the world is only as detailed as it needs to be for this one story. Nothing more, nothing less. Most of it is left undefined. Because it didn't matter for this single book. And that can be your campaign. Just enough for a single story.
When you get a premade setting, ideally, it can handle many, many tales. So, it needs to be massive, indepth, broad, but with missing pieces and mysteries to uncover. It's basically a show bible (not a religious thing, it's a Hollywood term) for your campaign. And referencing and retaining all that info is immense at times, especially as the realities of a 40hr work week and a social life compete for your time and memory. Absolutely have forgotten key details because work was bad that week or that one time I decided to run even though my boiler broke and leaked everywhere the night before. Bad idea that one. Fried brain.
So, it does really save you any work. It just makes that work different. You simply read something instead of creating it yourself. Frankly, pretty much the same with using a licensed setting.
Probably not saying anything novel. I have had an insomina attack this week and haven't gotten to bed before 3am for 3 days. Fried brain. But, felt like sharing.