What is a significant reason that jets do not operate at very high altitudes?

Operating at very high altitudes presents a phenomenon known as "coffin corner." This term describes a specific flight envelope where the aircraft is at risk of entering a stall due to low airspeed while simultaneously being close to the limit of the maximum operating altitude due to decreasing lift. At these altitudes, the margin between stall speed and maximum operating speed narrows, which can lead to a situation where it's difficult for the pilot to maintain control of the aircraft.

The combination of these factors creates a precarious situation that can lead to reduced safety margins. Consequently, airlines prefer to operate at altitudes where they have a comfortable buffer between these critical flight parameters, prioritizing safety and efficiency.