I have read these questions:
In reality, they fuse the hydrogen nuclei (the cores of the atom) together into helium nuclei.
https://solarsystem.nasa.gov/genesismission/gm2/mission/pdf/NeutronStars.pdf
Since the neutron star is the densest object we know, it should have enough gravity to fuse the neutrons (made up of quarks), and create the heavier elements.
Question:
I think your question is asking why, at the ultrahigh densities of a neutron star interior, don't the constituent neutrons (and a small fraction of protons and electrons) fuse into an enormous nucleus?
Actually, something like that does happen in the outer, lower density regions of a neutron star. The presence of an ultra-relativistic gas of degenerate electrons inhibits beta decay, and allows the formation of very large, neutron-rich nuclei, with atomic masses of several hundred. Larger nuclei become unstable to beta decay and/or fission.
At slightly higher densities, the equilibrium structures can become long chains and sheets, held together by the strong nuclear force, known as "nuclear pasta". The fission of the pasta is again suppressed by a "sauce" of degenerate electrons, but the increasingly dense fluid of surrounding free neutrons eventually pulls the nucleons out and dissolves these structures at higher densities.
However, if you increase the bulk density still further - towards the density of atomic nuclei - the configuration with the lowest energy density is for the majority of the nucleons to be free neutrons, with a few percent of free protons and electrons. Large nuclei cannot be built because the surrounding sea of nucleons can easily cause fission and dissolution of any structure.
