I'm not sure if I understood your question correctly, because I think I just answered it. If omniscience is "inactual" it might still be possible if an omniscient entity did exist but stopped being omniscient. If you meant that an omniscient being never existed and wanted to ask why I think it's impossible for one to exist, I don't really have a good answer.

I suppose then the question becomes if it's possible that omniscience comes out of nowhere. It sounds to me that by the nature of omniscience, it should be possible because why would that be able to limit omniscience? But I don't see how this mechanism would work without being applicable to literally anything. By this logic, I could become omniscient in two minutes. Or you could. Or mine or your username could.