The process you're referring to, where electrons combine with protons to form neutrons, is called inverse beta decay or electron capture. It can occur under certain conditions, such as in the extreme environment of a neutron star.
In normal atoms, electrons occupy discrete energy levels around the atomic nucleus and are bound by the electromagnetic force. The combination of protons and neutrons in the nucleus is determined by the balance of the strong nuclear force and the electromagnetic force.
However, in the extreme conditions of a neutron star, the tremendous gravitational pressure compresses matter to an incredibly high density. Neutron stars are incredibly dense remnants of massive stars that have undergone a supernova explosion. The pressure is so immense that the electrons are forced into the atomic nuclei, where they merge with protons through the process of inverse beta decay.
In inverse beta decay, a proton in the nucleus captures an electron, resulting in the conversion of the proton into a neutron. This process releases a neutrino, which carries away the excess energy and helps conserve overall energy and momentum. The conversion of protons to neutrons allows the matter in a neutron star to become even more tightly packed and dominated by neutrons.
In normal life and everyday matter, the conditions necessary for electron capture to occur are not present. The electromagnetic force between the electrons and protons in atoms is usually strong enough to keep the electrons in the electron cloud surrounding the nucleus. Additionally, the energy required for electron capture to happen in stable nuclei is not typically available. It is under extreme conditions of high density and pressure, such as those found in neutron stars, that the conditions for electron capture become favorable.