Let me rephrase -
"how is it sensible to talk about a computer program without a programmer?"
"how is it sensible to talk about a computer without a builder?"
Pretty much the same way we can talk about a nuclear reactor coming into existence without a nuclear engineer. If these things can arise spontaneously...
You are happy with the idea that computers can simulate computers?
Are you OK with the idea that a sufficiently large computer can start from a random initial state and end up simulating a computer, maybe a different kind of computer, in some part of it's structure?
At the basic level this means that turing complete structures can arise within other turing complete structures.
So the question you are asking boils down to whether Turing complete structures can arise spontaneously - and not from a Turing complete structure i.e. without someone setting up the program.
Well there is nothing known to stop it from happening. These structures appear in Nature without human intervention.
As usual with these sorts of arguments it is not a disproof of an Ultimate Programmer, but a demonstration that there is no need for pre-suppose one when we do our physics.
However: there does not appear to be any reason to presuppose that this Universe is a simulation either. It amounts to a wild speculation that is useful for philosophy but not for physics.
It is part of a class of speculations - if our Universe is embedded inside another, what is that embedded in?
It's either Universes all the way up - or we just accept that this one is the top-level.