my credence

Why do you talk in terms of credence? In Bayesianism your belief of how likely something is is just a probability, so we're talking about probabilities, right?

I am not saying that my learning information about them is actually changing the odds, just that it is giving me more information with which to make my knowledge of the already-existing world more accurate.

Sure, OK.

Now to bring it back to the point, what are some obstacles to your credence to thinking you are in a simulation?

Aren't you doing some rather severe privileging of the hypothesis?

The world has all kinds of people. Some want to destroy the world (and that should increase my credence that the world will get destroyed); some want electronic heavens (and that should increase my credence that there will be simulated heavens); some want break out of the circle of samsara (and that should increase my credence that any death will be truly final); some want a lot of beer (and that should increase my credence that the future will be full of SuperExtraSpecialBudLight), etc. etc. And as the Egan's Law says, "It all adds up to normality".

want to create simulations with pleasant afterlives

I think you're being very Christianity-centric and Christians are only what, about a third of the world's population? I still don't know why people would create imprecise simulations of those who lived and died long ago.

If some humans want to make simulations of humans, it is possible we are in a simulation made by humans. If humans do not want to make simulations of humans, there is no chance that we are in a simulation made by humans.

Locate this statement on a timeline. Let's go back a couple of hundred years: do humans want to make simulations of humans? No, they don't.

Things change and eternal truths are rare. Future is uncertain and judgements of what people of far future might want to do or not to do are not reliable.

How could we be in a petri dish? How could we be NPCs in a video game? How would that fit with other observations and existing knowledge?

Easily enough. You assume -- for no good reason known to me -- that a simulation must mimic the real world to the best of its ability. I don't see why this should be so. A petri dish, in way, is a controlled simulation of, say, the growth and competition between different strains of bacteria (or yeast, or mold, etc.). Imagine an advanced (post-human or, say, alien) civilization doing historical research through simulations, running A/B tests on the XXI-century human history. If we change X, will the history go in the Y direction? Let's see. That's a petri dish -- or a video game, take your pick.

When we use what we know about human nature, we have reason to believe that people might make simulations.

That's not a comforting thought. From what I know about human nature, people will want to make simulations where the simulation-makers are Gods.

that there might be a large minority of people who will ‘act out’ creating simulations in the hope that they are in one

And since I two-box, I still say that they can "act out" anything they want, it's not going to change their circumstances.

The difference here is that part of the belief in “Am I in a simulation made by people” relies CAUSALLY on whether or not people would ever make simulations.

Nope, not would ever make, but have ever made. The past and the future are still different. If you think you can reverse the time arrow, well, say so explicitly.

because I am aware that lots of human civilizations

Yes, you have many known to you examples so you can estimate the probability that one more, unknown to you, has or does not have certain features. But...

more reason to think that a different set of humans have done this already

...you can't do this here. You know only a single (though diverse) set of humans. There is nothing to derive probabilities from. And if you want to use narrow sub-populations, well, we're back to privileging the hypothesis again. Lots of humans believe and intend a lot of different things. Why pick this one?

Do you still not think this after reading this post?

Yep, still. If what the large number of people around believe affected me this much, I would be communing with my best friend Jesus instead :-P

why and how this has been frustrating

Hasn't been frustrating at all. I like intellectual exercises in twisting, untwisting, bending, folding, etc.. :-) I don't find this conversation unpleasant.

re: “cult,”

Nah, it's not you who is Exhibit A here :-/