Why do hot air balloons travel at low altitudes?

Answer 1

They fly at low altitudes because if you fly too high, the balloon may pop.

A2: Above seems unlikely, as the balloon is open at its lower end. Mainly I think because they are generally used for passenger viewing flights and the general public would not have the clothing for high altitudes as well as not wanting to be very high since the purpose of the flight is to view the terrain below. The balloon works by making the air inside hotter and less dense than the surrounding air, so as it rises it needs to use more fuel to achieve this as the surrounding air gets less dense. There must be an effective max height for hot air balloons but I don't know what the record is.