Imagine that you are a very creative Jedi/Sith and you are fighting against somebody of the opposite side. What's stopping you from killing him through the Force?
There are infinite ways of doing it more efficiently than through lightsaber fighting and nobody ever noticed or tried using, for example:
One of the first skills any Jedi or Sith was taught was called Force Resistance, also known as Force Shield or Force Guard--the Force equivalent of learning to properly root oneself or control one's center of gravity before a fight. This provided a basic resistance to any direct assaults on them through the Force, though an enemy could overcome it with additional effort. Hence a Jedi might be caught off-guard if ambushed and be force-pushed just as any regular being would be, but experienced force users would virtually always have at least some level of passive protection, just as veteran martial arts practitioners tend to keep themselves well balanced and grounded.
That said, there are plenty of ways to use the Force offensively, though the Jedi generally avoided such direct and lethal techniques. Vader could famously choke people even across great distances, and many force users, both Jedi and Sith, used a similar power: Force Crush. Note that, even here, light-aligned users tend to only use the power against droids or non-organics, while Sith had no such compunctions (though Mace Windu did use it against Grievous, Mace Windu was unique among the Jedi for his technique of skirting very close to the dark side but staying in the light). The power had other variants as well: Force Kill and Wound both appear in numerous games and media, perhaps most notably in the RPG video game Knights of the Old Republic.
Ultimately, why were these techniques not more commonly used? Well, apart from being passively/actively resisted by other force users, even non-force-sensitive beings had some low level of resistance, as the force flowed through all beings, whether they were aware of it or not. But even then, doing something like pinching a particular nerve or stopping minute signals in the brain would require exceptionally fine control, something Sith users rarely focused on. They would generally rather focus on raw strength, and besides, watching someone create lightning from their hands or crush someone's chest is probably more intimidating than simply seeing someone drop dead, regardless of what Rick Sanchez would argue.
