According to the Return of the Jedi novel where Luke constructs his first lightsaber, he uses the force to detect imperfections in the crystals and how to align them perfectly in the lightsaber. Technically, the force is not required to build a lightsaber. It's just a ritual part of the Jedi training to become intimately familiar with the weapon and how all the components fit together.
In one of the Timothy Zahn novels, I recall Luke saying that in his research, Jedi in the clone wars were able to speed up the construction process and build a lightsaber within a day during a time of crisis when the weapon was more important than the ritual (which took considerably longer than a day).
Ideally, a Jedi took many months to construct a single perfect weapon
that he or she would keep and use for a lifetime. Once you build it,
the lightsaber will become your constant companion, your tool, and a
ready means of defense.
While anyone can build them as they are just pieces of technology, not everyone can handle them. In most cases, force training is required to properly handle the weapon. Which is why not everyone has them (that, and the Jedi and Sith tend not to share the building plans with non-Jedi/Sith).
As for Grevious, the lightsabers were trophies.