The wording implies an offense has been committed, but this is not the case.
For one, because it's false: progress happens in book in Discworld. There's a series of "industry/progress" novels that illustrate change in the Discworld.
But also, because the timelessness of Discworld is what makes it appealing. With some exceptions, you can read a lot of the novels in any order. This makes them good, not bad.
Some certainly fit into that pattern (I’m thinking specifically Soul Music and Moving Pictures) but things do evolve in the interconnected series.
Especially considering the later ‘Industrial Revolution’ stories - the Truth, Going Postal, Making Money all made some incremental changes to Ankh Morpork. The last (and sadly one of the worst) books Raising Steam had the biggest potential to change the fantasy world with the introduction of railway travel.
Edit: and the police.
Isn't that the case for most people, that they don't end up changing the world or society around them? At best maybe they helped open a hole in the dike for water to seep through for future generations.
Obviously that's not the case for superhero or chosen one stories, but true for most people in life.
Homer is dead, we live in linear time, can we please please have some progress for a change?
A notorious offender is Terry Pratchett's Discworld. When the world needs to change, it happens between the books. In any book, you know you will end up with the same status quo as when you started reading.
Even if we are talking about something happening off-world, the experience would completely change the character. They can't just resume being young, shy and ignorant. Even some PTSD would be better than nothing.