Demigods are demigods for the simple reason that the Gene Level evolution from rank-13 to rank-14 was not just a simple step up.
A Rank-14 Gene Body contains ten large gene chains, whose spiral structures are the final state of a gene chain's linear structure. Another step forward would change the linear structure of said chains dramatically and by then, humans would no longer be humans. Instead, they would be deities.
Due to the extreme stability and perfection of a demigod's gene chain, those who attain that level would attain a level of combat effectiveness a hundred times over one who was on Rank-13.
That aside, one would also be able to ignite Ultra Fire within your body and begin modifying your cells if you had a spark at this stage. However, this process would usually require a long time before completion.
