Why do airplanes fly in stratosphere only?

Commercial jet aircraft fly in the lower stratosphere to avoid the turbulence which is common in the troposphere below. ... They are found at altitudes of 15 to 25 km (9.3 to 15.5 miles) and form only when temperatures at those heights dip below -78° C. read more

Most airplanes fly in the troposphere just 1 layer under the stratosphere. They fly there because the air is thin enough to minimize drag but enough to generate lift. read more

Although the altitude of the tropopause (the lower boundary of the stratosphere) varies depending on latitude, jet airliners, cargo jets and military jets often fly in the stratosphere, since the thinner air causes less drag than the thicker air down in the troposphere. read more

Commercial airliners typically cruise at altitudes of 9–12 km in temperate latitudes, in the lower reaches of the stratosphere.[2] They do this to optimize jet engine fuel burn, mostly thanks to the low temperatures encountered near the tropopause. read more