I think Ben’s post does a pretty good job explaining things, but to elaborate a bit on the fun of being an old school GM:
I like being able to reveal all the cool the prepared stuff to the players. Yes, discovering this stuff as a player is fun, but also knowing what is there on the map and then when the players go there, you get to show it to them, is also fun. And just because you’ve made the map and made dungeons (or picked ones by other people that you like) and placed them on the map, that doesn’t mean you know what is going to happen. The players still make choices (and rolls) and there are still random tables to provide the uncertainty. But enjoying this role requires you enjoy setting an impartial stage for the players and give up on discovering the setting. If you’re not into that, this is probably not the kind of DMing for you.
Being a DM and rolling on a random table, and being a player discovering a pre-designed map that the DM reveals, are absolutely NOT the same kind of fun. As a player, knowing the thing we are exploring is not improvised, has already been made and will not be changed by the DM on the fly, is an important part of the game and is not something the DM of that game can experience. We can all have fun with random tables on top of that, sure. And you can run a game with a blank hex map and a stack of random tables and just generate the setting as you go (and that might still be considered OSR or old school) and that can be super fun, but it is still not the same experience as exploring a prepared map.
So if you are going to DM a prepared map, you have to enjoy the reveal. That’s pretty much core to the job. A few other enjoyable aspects I can think of:
It is much easier to contextualize random encounters when you know the full setting. Something generated on the fly does not have the same potential for cohesiveness as a pre-designed setting does (unless you are playing a character-focused game that is not about exploration/discovery). It is harder to connect a random encounter to a larger map, situation, regional power struggle, etc. if that thing does not exist yet.
When you are improvising stuff (setting, situation) for players, it can feel bad when it flops. You have an agenda to make things exciting, or genre-appropriate, or connected to the players’ characters, and if the players are bored for a while and your material falls flat, it can sometimes feel like wasted time. But if a prepared section of a dungeon is kind of boring, it’s not so bad. You get to run it impartially, and players still get the feeling that they accomplished some exploration. And now they have the knowledge of this section and they can use it as a resource later on. Then again, if your players don’t feel like exploration on its own, for its own sake, is a fun thing to accomplish, maybe this is not the right style of game for them.
And finally, even though you don’t get the same experience of discovery, you get a thing that the players don’t. When they start tearing it up and wrecking things and making waves, if you have made a large, interconnected setting, you get to see the players affect parts of it that they don’t even know about yet. Sometimes this is bad for the PCs, sometimes really good, and the players don’t even know what the repercussions of their actions are going to be, but you do. And you get to think about them, and contextualize them, and then reveal them just like any other part of the setting.