Because they are hard to make. Oh, not the graphics, or the engine, or anything like that. That shit’s easy (for AAA studios with access to hordes of outsourced studio labor.)
They’re hard to design. Good immersive sims have so many options, and threads, and interlocking systems. It takes actual, honest effort, from talented, passionate individuals, to come up with and wrangle all that neat stuff.
And you can’t crank them out quickly. They have to cook. They have to iterate. Hell, sometimes entire systems just don’t work and have to be torn out and started over. They take time.
And all of the above costs money, for games that won’t return obscene profits from microtransactions.
All of which is completely antithetical to the current AAA régime.