Why don't commercial airplanes carry Earth-observing instruments?
They do! Well, at least some of them. There is for example a project called TAMDAR (Tropospheric Airborne Meteorological Data Reporting) that e.g. Icelandair is a part of. There is a document published by the Icelandic Meteorological Office about it.

Another example is MOZAIC/IAGOS: Atmospheric Research Using Commercial Airliners.