Single player is a different kind of experience, and no less valuable. You might as well say

> It's a lot harder to make a book have depth and complexity than it is for going to a party, since you don't have human conversation partners.

It depends on the book and the party. Similarly, maybe there is more depth and complexity to Dwarf Fortress than there is to Rocket League. (Not to pick on RL in particular, it is just the first thing that came to mind.)

I'd say Breath of the Wild and Tears of the Kingdom had a lot of depth. There was a ton of stuff to do. I have 340 hours into Tears of the Kingdom and hit 100% on the map, but there are still things I haven't done and stuff to explore and try. I find they also have a high replay value, since they are so open and there are nearly infinite ways to solve the various puzzles, traverse the world, or engage in the various battles... or don't. I once started up a new BotW game to see how far I could get without actually fighting anything.

I’d say it’s the opposite. But you are probably talking about depth of combat skill, while I’m talking about depth of writing and story.

I think it's highly dependent on the type of game. Games that involve planning and strategy like Slay the Spire or Factorio have enormous depth despite being single player. But I think that it's hard to make the actual execution of mechanics as fun or deep against computer opponents.