The last digit of pi doesn't exist since it's irrational. Chaitan's constant, later busy beaver numbers, or any number of functions may be uncomputable, but since they are uncomputable, I'd be assuming that their realizations don't exist. Sure, we can talk about the concept, and they have a meaning in the formal system, but that's precisely what I'm saying: they don't exist in this world. They only exist as an idea.

Say for instance that you could arrange quarks in some way, and out pops, from the fabric of the universe, a way to find the next busy beaver numbers. Well, we'd be really feeling sorry then, not least because "computable" would turn out to be a misnomer in the formalism, and we'd have to call this clever party trick "mega"-computable. We'd have discovered something that exists beyond turing machines, we'd have discovered, say, a "Turing Oracle". Then, we'd be able to "mega"-compute these constants. Another reason we'd really feel sorry is because it would break all our crypto.

However, that's different than the "idea of Chaitan's constant" existing. That is, the idea exists, but we can't compute the actual constant itself, we only have a metaphor for it.