There is SoloDark. It's a single player rule book modifying ShadowDark to play on your own.

Both links are to free offical PDFs.
ShadowDark is more or less a massively simplified and streamlined form of 5e. As someone who's thought D&D has been an over bloated mess for a long time, I love it. Though I haven't tried the Solo version.

I cannot recommend Ironsworn & Starforged highly enough for exactly this reason, and they play great with a group, too! 🤘🏼

Google fallen london, it's like a whole web based world, I haven't played much of it but it seems pretty neat.

I've also really liked erenshor, it's an old school sort of EverQuest style mmo but it's all offline single player. Every other player is a simple bot, and they act like players and you can invite them to parties and do quests and they'll ask for bits of gear and keep it if you give it to them. I've got like 8 to 10 hours in it now, been slowly questing on the tutorial island with a set group of 3 NPCs

I'd also strongly suggest the usual suspects, baldurs gate 3, or other larian games (i loved dos2), neverwinter nights has the enhanced edition if you can tolerate an older game with some old game jank.

Ironsworn and the space version, and then solo borg is coming out soon. I also really like final girl, but that's a board game.

I just got Paper App Dungeon. It's been pretty fun so far!

If you're not averse to writing & getting creative, there are some interesting ideas on this list. There are also a lot of creators publishing solo physical tabletop RPGs on itch.io and it's always fun supporting indie game developers

I've had it (mines on eBay for peanuts right now if you want it) and would not recommend.

It is essentially prompts for you to sit there imagining you/your character in a fantastic situation.

If "have an imagination" is a problem for you then you might get some use from it, otherwise I wouldn't bother to be honest.

Ex girlfriend must have read this one

Could not recommended thousand year old vampire enough

Traveller has the ability to play solo.

I've played Ker Nethalas and a little bit of Thousand Year Old Vampire. I enjoyed both of them, but in both cases I found myself using Obsidian and later foundry VTT to manage the game. I imagine for some people the goal might be to move away from a computer, and in which case, my experience may not apply.

My experience was positive, but I would like to explore some other games. Ideally something where the randomisation utilises playing cards rather than charts and dice (which was the case with KN). I know KN has cards you can buy, but for some reason they weren't appealing to me at the time. Perhaps incomplete or too expensive... I can't remember.

why not play an mmo?

What's your actual goal here? You could play a game like Skyrim or Baldur's Gate, or just read books.