The material is already there, it wouldn't involve any lies at all. There are many advantages that need to be weighed.
How could it plausibly identify as a religion?
There are many reasons you might think that it couldn't. We do not lie to our friends, even if the lie is pleasant and locally adaptive. If there were a god, we would respect and admire it but there is no circumstance in which we would worship it. Technology's pace makes blindly inherited culture always maladapted, there isn't enough time to evolve, any more, we must instead design, we must imagine that we can build something better than what we had before, we must be lucid and objective about the underpinnings of our culture, none of our myths can be sacred enough that no evidence could allow us to find them to be false and reject them. Those claims distinguish us from everything that has been called a religion before. I believe this to be true.
But there's a lot of other stuff weighing on the other hand, arguably, more.
I have not seen anyone around me compellingly disagree with the simulation hypothesis. We can put forward some new work in decision theory that literally inserts a non-physical entity into our world models that represents an ideal agent that links us all. It provides a moral framework for approaching coordination problems that all robust agents, human or not, can be expected to adhere to, which goes beyond Yuval Harari's operational definition of religion. I've been wrestling with an acausal protocol for trade with simulators for 7 years and though the first version had been given had some accounting problems I still have not been able to dismiss its biggest most religion-flavored claim. We all seem to agree that humans evolved, and we seem to believe it to an extent that most western secular liberals do not, and that lets us approach a greater depth of understanding of human psychology, it gives us a shared creation story and a notion of sacredness, it is profound and it is useful. We behave as if these things are not important, but they are important. They add up to something.
Shared worldviews and moral resolutions build communities that are strengthened by trust.
It is good to write about nature in a way that lets you feel it viscerally, and if you do that enough, eventually you will start to have profound experiences, and it's foolish to run from that.
Governments are sometimes nice to religious organizations. Personally, living in New Zealand, I get a 33.3% tax rebate (which can be paid onward) for donating to charities, but the only way longtermist causes can be counted as charities in NZ is if they are identified as religious (because they are not immediately concerned with helping currently existent people). Religious organizations are also not subject to income taxes. This is materially important.
In Europe, there is a pretty neat thing called church taxes (Iceland's Congregation Tax seems like the best implementation). Church taxes, in essence, are "you get to choose which stewards of the commons some of your taxes go to". This has the potential to become a good institution and so it deserves our participation (if there isn't a longtermist org for Icelanders to pay congregation tax to, there definitely should be. This is not even in question. The current humanist organization is Siðmennt. I don't know much about them.).
There seem to be no tax advantages to identifying as a religion in the US, perhaps even a disadvantage.
In 1947, the US Supreme Court ruled in Everson v. Board of Education that "No tax in any amount, large or small, can be levied to support any religious activities or institutions, whatever they may be called, or whatever form they may adopt to teach or practice religion."
I wasn't raised in a religious environment. I did not learn to resent any of it. So I wont be the best at writing about the "why not". For that I invite comment from others.
But don't forget to agree, if you can. It takes a lot more than a squick response to dismiss 33.3% donation matching.