Air launch (to orbit)[1] has been a thing for a good while now. As others have stated, fuel savings are enormous; jet engines have orders of magnitude higher efficiency than rocket engines when taking fuel tankage into account. Air resistance is barely an issue, and same for the gravity drag. But most importantly, launch mass is greatly reduced in part thanks to not having to carry oxidizer for the first stage. Another factor, rocket's first stage must have powerful, thus heavy, engines with enough thrust to both counter gravity and impair acceleration; an orbital insertion stage can do with much lower TWR, thus again mass savings for air launch.

Point in case, note the size of the Pegasus orbital rocket [2]. Or the orbital anti-satellite missile, sized like a typical cruise missile, all thanks to air launch [3] from a typical fighter jet.

Now that SpaceX is able to recover the first stages from some launches, the cost of first stage is a bit less of a factor, but still there are other considerations, like ability to launch from any decent runway vs having to maintain a dedicated facility, unusable for any other commercial purposes, far away from humans due to incredible amount of noise rocket engines create.

[1] https://en.wikipedia.org/wiki/Air_launch_to_orbit

[2] https://en.wikipedia.org/wiki/Pegasus_(rocket)

[3] https://en.wikipedia.org/wiki/ASM-135_ASAT