One way to think about it:<p>They do decay into protons and electrons, and then are immediately squished back into neutrons by the immense pressure. (There is a missing neutrino to wonder about though.)<p>Another way to think about it:<p>They are unable to decay into protons plus electrons because there isn&#x27;t enough &quot;room&quot; for the two particles to exist in the space formerly taken up by one particle. See: Electron degeneracy pressure.<p>Basically all particles decay, unless there is something stopping them - for example there is nothing lighter for them to decay into.<p>You could ask why don&#x27;t the neutrons in nucleus&#x27;s decay into protons? It&#x27;s because it&#x27;s not energetically favorable for them to do so.<p>That&#x27;s basically all decay is: Is it energetically favorable to be something else. The more energy is involved the &quot;faster&quot; it decays (sort of, it&#x27;s more like the more energy relative to the barrier).