Well yeah; I love playing the various mini-games -- character generation, designing ships, world-building, writing scenarios. People who do not love doing those things can always fall back on what others have done; but someone, somewhere has to put in the effort -- whether it is 'for love of the game' or to sell a one-shot through DriveThruRPG.
Players can define the universe too; they can design equipment, ships, worlds, NPCs, cultures, etc. Although they cannot put them directly into the particular universe they are gaming in, they can submit them to the GM and watch as they seep into the adventures they are playing in. Again, someone has to put in the effort, and the referee has to be familiar with it and choose to put it in.
Most people use "prep" to mean work that goes into planning for a particular session. Saying that I know how
Traveller or
Ars Magica or whatever work well enough that I don't need to put specific work into specific sessions is still "prep" kind of makes any conversation about prep moot. It doesn't really mean anything at that point.
There are a few games, like highly mechanically complex ones such as
Champions, or very dungeon crawl focused ones that would probably need session specific prep because of the nature of the game. While there are others like
Blades in the Dark where session specific prep is anathema. Most games are in between.
Yeah, I like to world build. I work on all kinds of game stuff for various games because I enjoy that activity, whether I expect to ever use them or not. I don't consider that "prep" because it isn't prepared for any particular application. Sometimes I get to use it in a game. That's bonus.
I started playing Traveller in 1979. One of the first books I got (other than the core rules) was
76 Patrons. That, and the Amber Zones in JTAS, are the way I learned to "prep" Traveller adventures. A sentence or three of situation. A few ideas for different twists on the ending. Nothing much in between, because who knows where the PCs are going to go or what they are going to try to do.
If the players decide to just go gather rumors amongst the salvagers and I throw out a lead to a derelict ship, I personally don't need a mapped out derelict ship planned. I know what ships look like in Traveller and the players aren't likely to have schematics anyway. So I can sketch out a ship as needed basically on the fly.
But that's me. That's how I learned to run games and what works well for me. I don't expect that to be what works for other people. I don't tell people to stop prepping if prepping is working for them and they are having fun.
To your earlier point, yes, if I actually picked up an adventure and told people "Hey, we are running
Secrets of the Ancients", I would indeed feel like I should read that adventure and run it reasonably faithfully (subject to whatever customization needed for the group of people playing). But I've only ever encountered a few adventures where I thought I might want to do that. (
Bayern for
MgT2300 and
Six Seasons in Sartar for
Runequest).