Preparing campaigns and games is what GMs do, and in solo you are always the GM. So, it should follow that if anyone is doing the prep, it will be you.
My problem with prepping solo games is that everything you prep in advance you are likely to want to use, and that cuts down the available space to improvise in.
We can become very attached to our ideas, even more so if we spent a lot of time developing them, creating maps and NPCs, and all kinds of spin-off side quests.
Once you become attached to something it is harder to let go. One of the things that make solo play really cool is that you don’t know where your adventures may take you. When you have something you really want to include, because you have prepped it all, that can come into conflict and work against this open-world goal of oracle-driven play.