Short answer:
"A Jedi uses the force for knowledge and defense, never for attack." -Yoda
Long answer:
In the films, Jedi are rarely shown to use their powers while fighting. Battles between Jedi consist mostly of lightsaber dueling.
- When Obi-Wan duels with Vader in A New Hope, they rely entirely on their lightsabers. Neither uses any kind of Force power.
- When Luke duels with Vader in The Empire Strikes Back, neither makes significant use of Force powers. The only time one of them uses the Force in an aggressive manner is when Vader hurls objects at Luke. He does this from a distance rather than while they are clashing with their sabers.
- In the conflict between Luke, Palpatine, and Vader in Return of the Jedi, (if I remember correctly) Luke never uses the Force as a weapon, while Vader performs a Force-enhanced saber throw once. Palpatine does use his Force Lightning against Luke, but he does this after Luke has already effectively surrendered
- In the conflict between Qui-Gon, Obi-Wan, and Darth Maul in The Phantom Menace, the three combatants rely mostly on their lightsabers and acrobatic moves such as jumps and whirls. I think we see a few Force-enhanced jumps, and possibly using the Force to throw someone off balance, but never any significant use of the Force as a weapon
- I don't remember the battles in the next two prequels as clearly. I think Dooku makes limited use of Lightning in combat
- In the duel between Rey and Kylo Ren in The Force Awakens, both combatants rely almost entirely on their lightsabers. I don't recall either using the Force as a weapon
Although every film except Rogue One includes at least one lightsaber duel, we rarely see Jedi on either side using the Force itself as a weapon. When we do see a Jedi using the Force in an offensive way, it is usually from a distance while having a significant advantage and while not using a lightsaber.
This is enough evidence to conclude, at least as far as the films are concerned, that it is probably not practical to use Force powers offensively when engaged in a lightsaber duel. It seems reasonable to assume that lightsaber combat requires too much concentration for Jedi to use offensive abilities at the same time. Taking your attention away from your lightsaber long enough to use another power might give your opponent a momentary opportunity to attack (in the game Jedi Outcast, activating a Force power often left the user momentarily more vulnerable; if I remember correctly, one of the better strategies was baiting your opponent into using Force Push and then throwing your lightsaber through his torso as he activated Push).
While Grievous does not have the Force, he is wielding four lightsabers, which may be more than enough to require Obi-Wan's full concentration. It's also possible that Obi-Wan's thinking is more limited due to his training. Let's assume that Obi-Wan was trained that it is not safe to use the Force in an offensive way when saber-dueling another Jedi, because the momentary break in concentration will leave him open to attack. If this has been ingrained into him since he was a child, it might never even have occurred to him to use the Force itself against Grievous, even though Grievous would likely be more vulnerable than another Jedi.
