> What makes you think that?

Because if it's physically grounded then you can build a physical analogue of it.

> overwhelming majority of functions aren’t computable

By Turing machines, not by any other type of machine. As I said, if hypercomputers exist then they can solve the Halting problem, which Turing machines cannot.

Consider the set of functions that that map the reals onto other reals. Almost all of these functions are truly random, with no way of expressing them that does not require the storage of an infinite number of infinite strings.

Not only is there no practical way of creating such a thing, most formulations of physics preclude any possibility of making one by placing finite limits of the amount of space or time accessible to us.

(Not to mention that almost all reals are [Turing] uncomputable in their own right, but that's a more complex thing to demonstrate.)

You don't need to express or encode the reals if you've built a physical analogue that operates on those reals, that's the point.