system
Given the following post, respond as if you're commenting on Reddit:
human
Why do commercial aircraft fly against the jet stream?

I've noticed (in the USA) that flying from west to east is significantly faster than flying from east to west.  This is because of the jet stream.    For instance, flying from Philly to Denver will take about 4 hours and the aircraft will manage a ground speed of about 440mph at 36,000 feet.  From denver to philly takes about 3 hours, the aircraft will average a ground speed of about 550mph.  I've also noticed that if the aircraft stays down around 27,000 feet, it can stay out of the Jet stream (or be in a slower part).  So why don't planes heading west fly lower to stay out of the jet, save fuel and time?
gpt
Many of them do depending on the air streams for that particular flight. However weather and other air traffic will dictate the altitude and course that particular aircraft takes. For instance an aircraft heading west might be scheduled for 27,000 feet but half way through its journey there may be a storm that would create some considerable turbulence. To avoid the turbulence or "chop" said aircraft may climb several thousand feet to get above the rough air.    Airlines have spent decades studying and planning  the fastest/most fuel efficent routes for their aircraft to take. This helps them since their motto lately seems to be "fuel costs money"