According to https://en.wikipedia.org/wiki/Sphere_of_influence_(black_hole) the gravitational sphere of influence of a supermassive black hole is really limited compared to the size of its hosting galaxy, why then is there one at the center of every galaxy?
As some of the comments pointed out, there isn't a definite answer since this is still ongoing research. However, there are some things that might explain it:
Galaxy formation: Current theory says that dark matter is non-uniformly distributed in the early universe. Once radiation and baryonic matter decouple, matter flows towards the dark matter overdensities. We know that stars (Pop III) must form very early during this, because their light reionizes the universe. These early stars are thought to have a mass of up to 1000 solar masses. It's possible that some of the infalling gas collapses straight into a black hole.
Density profile: A galaxies' density rises sharply towards the centre. So, during the formation of the galaxy, the gas rushing inward might have formed a black hole without going through the process of star and supernova. Also, this means there's enough to feed the black hole once it's formed.
Dynamic friction: Imagine a self-gravitating system of heavy and light objects (like a star cluster or, indeed, a galaxy). A heavy object passing through an ensemble of lighter ones will lose some of it's kinetic energy to them, which means it will "sink" towards the centre of the gravitational well. This is well observed in globular clusters. Problem: The timescale for this to happen is far too long to matter for large galaxies.
There seems to be a correlation between the mass of the central black hole and the mass of the elliptical galaxy or the bulge of the spiral galaxy. So, some process makes sure that the central black hole gets to grow along with its galaxy.
Coming back to your question: There are two parts to it, one: why a black hole at all, two, why at the centre. The second part is simpler, it would get there eventually, and probably (let's say in a galaxy merger) quite quickly. For the first part, there is no definite answer but the points above are some indications as to why it's not too surprising that there should be one.
As a last point, black holes at the centres of other galaxies are not at all easy to find. Unless the black hole is active (quasar), there are at best very indirect detection methods (velocity dispersion in the central part etc.).
The gravitational "pull" of a black hole is not limited. In fact the gravitational force of attraction of every particle in the Universe effects every other particle in the Universe, irrespective of their distance of separation or mass. Or to be more exact every particle distorts the fabric of time/space throughout the Universe. The distortion may me small the further away from the particle you get but it is still there according to General Relativity.
Quantum theory will put a of limit on things, in that the when the distortion is reduced to the Plank length it will be lost in the general background noise.