I would think the antennas on cell towers are polarized to limit radiated power up.

Curvature of the earth means that an airplane may be horizontal at 20+ miles out, while a surface vehicle is blocked by ground.

Not so much polarised as aimed. I'm not familiar with 5G (which I believe uses phased array beam aiming), but earlier generations had provision to physically tilt aerials in the vertical plane to optimise beam direction.

They are.

Would that do much for multi path bounces etc?

Depends on the base station. We usually point them down and try to adjust the power output not to "spray" the signal all over the place but rather be focused (and save operator costs), however some stations are set on max power and a lot can bounce away, indeed.