Reading through this, I realise that 'preparation' means different things to different people. In
very broad brush terms, I would suggest that there are two types of preparation: World-building Prep and Adventure Prep. Of course they bleed into one another (since adventures generally take place somewhere in the 'world'), but they ca be handled independently.
It is clear that a fair number of people who do not prepare for a role-playing session nevertheless
have prepared, in the sense of reading about or developing their world-setting. I count myself in this group - I spend far more time working out the details of towns and countries than I do thinking of specific events for the PCs to interact with. I know my setting well enough that whatever the PCs decide to do I can probablt generate something 'realistic' and interesting for them to do. I think that this accounts for the sizeable fraction of people who don't prepare who run their own homebrew worlds. They've already put the work in, so it's really a matter of tapping a pre-prepared stockpile of goodies!
Adventure-prepping is the specific work of determining who and what the PCs are dealing with. This is most essential when dealing with purchased modules, since you do need to familiarise yourself with the story and setting unless you want to spend lots of the session going 'ummm.... errrr.... just reading it up now... he's an Elf! And a woman...'

Now, I do think that part of the difference in preparation for the adventure arises from experience. I know my players fairly well, I know the setting and I know the sorts of things they like to get up to. So I can get away with little prep. But change one of those factors and even after 20 years as a DM I can still get floored!
Whether or not you prep is partly a matter of experience, partly a matter of familiarity (with the PCs and with the rules). But it's also a stylistic choice. Some people simply find that they can't stand working from aen extensive adventure 'script' whilst others like the feeling of having sorted out the details in advance so that they
don'y have to suddenly decide where the nearst inn is, or who in fact received the message puch from the Royal Groom...
I would encourage DMs and players to experiment a little with how they prep for the gaming session. Different models suit different people, and can make for better sessions!
