system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are commercial airplanes all sub-Mach 1?
gpt
The faster you go, the harder it is to push through the air - and the relationship is not linear. So, going a little bit faster needs a LOT more power which means more fuel, bigger engines, more noise, etc. etc.

Put your hand out of the car window at 30mph - easy. Now try 60mph - wow, the air is a lot more forceful! Now try 120mph (OK, don't really) you can't hold your hand up, the air is like a brick wall. At 600+mph you can imagine it's not easy!

If you look at Concorde, it's a tiny cramped thing that burns massive amounts of fuel, which makes it expensive to run and you can't fit many passengers on it.

Additionally, stuff happens the faster you go - stuff heats up, shockwaves happen, the whole airplane has to be made to withstand the extra forces & strains, lifespans of components get shorter, stronger stuff is heavier and bigger, etc. etc.

*Please don't put your hand out the window and then sue me because you hit something*