Why are supermassive black holes at the center of galaxies?
Galaxies form in a much similar way to solar systems like our own.
When a solar system is formed, there is a huge cloud of matter. All of the particles in this matter begin to pull each other through the force of gravity. Typically a majority of these particles begin to stick to each other and due to the close proximity of the particles the kinetic energy increases, thus the heat increases. The reminder of the particles go through a similar process to form planets and other solar system bodies.
Black holes form in a way much similar to this, however, instead of individual particles, whole stars are smashed together to make something so dense that in an area smaller than that of a tip of a needle, several solar masses of matter are contained. This process is made even more extreme by the sheer number of stars in the center of spiral galaxies, resulting in supermassive black holes.