why then should not creationists posit models also

There's no problem at all with that. Of course, it must be a model that fits ALL the known measurements and be a testable model - i.e. it must be falsifiable. Abide by both of those requirements, and I'm quite happy for anyone to propose a model, creationist or not.

In my opinion, though, in order to ensure that it fits all the known measurements, its predictions must also be expressible with acute precision, so I doubt any viable model is possible without a mathematical component at its heart.