Looks like telekinetic force abilities could not be used by average Force user at will
Example 1: Yoda vs Dooku. Both are very powerful Force users, Dooku former Jedi Master and Sith Lord, Yoda Grandmaster of the Jedi Order. Yet, both of them required couple of seconds to actually control large objects. Dooku is much quicker with Force Lightning then with Force Push/Pull. Interestingly, neither of them tries to use Force to directly move opponent. Most likely because of Force resistance, ability to repulse opponent's attack with Force.
Example 2: Yoda vs Palpatine. Yoda manages to Force Push Palpatine at the beginning, but this is just one time event when Palpatine was not ready. After that they only throw objects at each other. Palpatine (probably most powerful Force user in the Galaxy) seemingly easy throws pods at Yoda. Yet, when Yoda with some effort hurls one pod back to him, looks like Palpatine's concentration is broken. He could not stop that pod and doesn't attempt same tactics again.
Example 3: Anakin(Dath Vader) vs Obi-Wan on Mustafar and later on Death Star. Here we have two not so powerful Force users. Anakin never achieved his full potential, Obi-Wan was more of average Jedi, with some luck and wisdom. They attempt to Force Push each other in one moment, but both Force Resist. Rest of conflict is usual swordplay with lot of jumps, but no other visible Force display. On Death Star not even that, both of combatants have aged and grown weaker. They do know to protect themselves from telekinetic attacks, so neither one tries anything
Example 4: Obi-Wan vs Grievous. Grievous is a living being, but not Force sensitive. So Obi-Wan does Force Push Grievous but only once. Why not more ? Most likely because Grievous is too heavy and powerful (good grip on any surface with his limbs and claws), and looks like Obi-Wan spent lot of his Force power for even that one effort. As I said before, Obi-Wan force powers were average, or somewhat above average. His telekinetic abilities are more of grabbing lightsabers (or pistols) than pushing large objects around.