the elevation gain is a marginal benefit compared to the velocity required. no plane is going to give a meaningful velocity as a first stage, and at best will give not nearly enough altitude to matter, while severely limiting the maximum mass of the rocket.

just for some context, the energy to lift 1kg to an altitude of 500 km is 4.9 MJ (being extremely generous and assuming constant surface gravity), while the energy to accelerate 1kg to a velocity of ~7.6 km/s (the required velocity for a circular 500km orbit above the earth) is 28.9 MJ. so even if the plane could magically get the rocket all the way to altitude, it only saves about 14.5% of the energy required to achieve orbit.

practically speaking its likely impossible for a plane to get past the karman line (100 km) without the assistance of a rocket engine, and according to wikipedia the altitude record for a jet plane is only 37.6 km, which is only 7.5% of a 500 km orbit. its just way easier and cheaper to make a bigger rocket than to bother with putting a rocket on a plane.