So what you're saying is someone made a single-player mod for it?
Sort of but it is complicated. Firstly, even the MMO version is probably something you could enjoy. The official version is really dumbed down and a total waste of time. But there is a semi-official fan-run version of it called Project 1999 which reproduces how the game was in 1999. It is much better. It is played online but you can play alone. Some of the classes can't cope very well alone and need to group with other people once you get a bit higher level. But some of the classes can be played alone all the way through the game. The spell casters like Necromancer, Magician, Druid, Enchanter, they can solo. Combat is a modified AD&D, almost everything has a dice roll and the spell lists are very similar etc. But the game is played real time, and it has a lot of depth and quite hardcore like an old RPG, some spells require components, you can be poisoned, diseased, etc. There are resistances to fire, cold, magic, disease, poison, etc. The game was designed for people to play with a group of other real people, but there was "emergent gameplay" where people figured out how you can play solo too. Most melee characters are simple to play but need to group with others to cope after level 10 or something. Spell casters are strong enough to play solo. Necromancer for example has the Fear spell which lasts about 30 seconds, but also an ensnare type spell, so if you use that first and then Fear, the enemy runs away slowly and you can follow it with your skeleton pet and start casting poison and disease spells on it. But there are other ways to do it and you need to use different methods for different areas etc.
Secondly, there is a thing called EQ Emulator which is another community created project which copies the original (great version) of the game into an exe that you can run and make your own server. You can run it offline so you can play on your own pc, and there are dozens of servers run by various people online, and each one has their own rules. So people who found the original game to take too much time have easier/faster versions of the game, and everything in between.
But here is the more complex part. I played the game for years by myself and it is extremely good, I occasionally grouped with other people but just soloing with a Necro or Enchanter was the best gaming I've ever had, especially doing dungeons by myself. But you can run the game a second time, and make another character, and now you can alt-tab between the two characters and you play them both. It takes a lot of getting used to but it gets twice as interesting and you get twice as strong, especially if you add a Cleric or something. You can do this 6 times and have an entire group that you control yourself, like Baldurs Gate 2 but no pausing, real time, and you have to alt tab to each character to do what needs doing. It is doable but very intense, so people get macros to jump to a specific character with a button press, and then there is a mod which basically lets you program the other characters to do what you want. So you play a Warrior and attack an enemy and then your Cleric will automatically heal you, your Druid will start poisoning things etc. It can get extremely complex but it is best to start simple and just do more when you want to do more.
I am convinced anyone who loves renaissance era RPGs would be amazed by it if they gave it some time. And you know RPGs better than anyone so I think it might really interest you.