On rocky planets gravity doesn't get much lower in the orbits we're concerned about. For example the ISS still experiences 90% of the gravity we experience at the surface. Reaching orbit is mostly about reaching a speed where the arc in which you are falling never intersects the surface.
But you can absolutely use an aircraft to gain height and speed, and then launch a much smaller rocket from that aircraft (where the speed is the primary advantage, and is what rockets use most of their fuel for). This setup is used by Virgin Galactic's SpaceShipTwo. There is also Virgin Orbit's LauncherOne, which is a small rocket that launches from a modified Boeing 747. On Earth it's just about not worth the additional complexity, but on planets with stronger gravity but comparable access to powered flight this might be the preferred method of reaching space.
One important factor might be the speed of sound. Subsonic flight is much easier for aircraft than supersonic flight. In an atmosphere with a much higher speed of sound, like say hydrogen, aircraft could reach much higher speeds and thus would be a much more advantageous launch platform for rockets. Assuming you already solved the issue of powering those planes of course.
An air-breathing jet engine doesn't need to carry oxidizer, which in a rocket is most of the propellant weight. It also has access to unlimited reaction mass, so it can be much more energy-efficient in producing thrust (it is more efficient to produce thrust by accelerating a lot of mass by a little, than by accelerating a little mass by a lot, but a rocket can't take advantage of this because it would need to carry all that extra mass. A plane can use ambient air for this purpose)
This all adds up to a plane needing to carry many times less mass to gain the same altitude and speed as a rocket, at least within relatively dense atmosphere.
A rocket on a typical orbital launch profile spends less than 60 seconds in air dense enough for jet engines to have good performance, so there is little to gain.
Pegasus is an orbital rocket launched from an aircraft, but it doesn't exactly impress with performance or cost-effectiveness. Just doesn't make much sense to operate a huge aircraft and design your system around it just to improve on the least important 10% of the flight.
But you can absolutely use an aircraft to gain height and speed, and then launch a much smaller rocket from that aircraft (where the speed is the primary advantage, and is what rockets use most of their fuel for). This setup is used by Virgin Galactic's SpaceShipTwo. There is also Virgin Orbit's LauncherOne, which is a small rocket that launches from a modified Boeing 747. On Earth it's just about not worth the additional complexity, but on planets with stronger gravity but comparable access to powered flight this might be the preferred method of reaching space.
One important factor might be the speed of sound. Subsonic flight is much easier for aircraft than supersonic flight. In an atmosphere with a much higher speed of sound, like say hydrogen, aircraft could reach much higher speeds and thus would be a much more advantageous launch platform for rockets. Assuming you already solved the issue of powering those planes of course.