Why haven't I seen more people running [[Hell's Caretaker]] in this deck?

Is there a reason Hell's Caretaker doesn't make the cut?

Or do people just not know about it?

There are better cards. Meren is a tight list. Remember, there's a lot of people who have been playing for a very long time. I've been playing longer than some players have been alive. A lot of us push commanders to their limit. If you are not winning or locking down the game by turns 2-4 you're not even scratching the surface of what others have done in EDH.

I had [[Hornet Queen]], [[Viscera Seer]], [[Dictate of Erebos]], and [[Blood Artist]] out with [[Solemn Simulacrum]] in my graveyard

Hornet Queen, dictate, and solemn aren't exactly the best cards.

Am I missing something?

yes

Is there a reason I shouldn't run it that I just don't see?

You're playing in a relatively "casual" meta compared to some.

Here's an example of my [Meren list]https://tappedout.net/mtg-decks/meren-reanimator-of-clan-nel-toth-1/) (of which I don't even consider competitive) and it's basically on another upper level of existence than most. In my primer I include a link to Riku's proper cEDH deck which is in a league of it's own.