What is the main reason a hot-air balloon cannot normally be steered horizontally like an airplane?

A hot-air balloon cannot normally be steered horizontally like an airplane because it travels with the wind.

A balloon rises or descends by changing the temperature and density of the air inside its envelope. Once airborne, it generally drifts with the surrounding air mass rather than pushing through it with an engine, propeller, or wings. The pilot can control altitude but has only limited direct control over direction.

Experienced pilots may find different wind directions at different heights. By climbing or descending, they can sometimes influence the balloon’s route, a technique known as navigation by changing altitude. This is useful but not equivalent to steering an aircraft on demand.

The lack of ordinary horizontal control is why balloon flights require careful weather planning and why landing sites are selected from likely downwind areas. A balloon’s burner provides vertical lift control, not forward propulsion.