system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why aren't commercial airplanes getting any faster?
gpt
Most big airliners fly somewhere in the ballpark between Mach 0.5 and Mach 0.8.  Going faster than that would require a few things:

First, you'd have to use a lot more fuel.  Drag increases ~~exponentially~~ geometrically (see comments below) with speed, so going faster requires you to burn a lot more fuel.  Fuel is the biggest cost associated with commercial air travel, so a faster plane would be a lot more expensive to fly.

Next, you'd have have to redesign the planes.  The current configurations of planes aren't equipped to travel at Mach-1 or near Mach-1 speeds.  If you look at something like the Concorde (a supersonic commercial airliner) you'll see that it has less room, as wide bodied designs aren't really conducive to supersonic travel.  

Assuming you want to go supersonic, you'd have to get around the problem with sonic booms.  Sonic booms are very loud, and most people get angry when a loud explosion happens over their house.  In the past, supersonic commercial flights were conducted almost exclusively over oceans because communities didn't want supersonic jets flying over them.  

_____

If you're asking why we don't go as fast as possible without breaking the speed of sound, it again goes back to cost and efficiency.  

If we have a 767 going Mach-0.75, a plane going Mach 0.99 is only 33% faster.  A flight that would take 3 hours would be cut down to 2 hours and 15 minutes.  Sure you'd save 45 minutes, but you'd burn considerably more fuel, which would drive up the cost of tickets.  

It's unlikely that the market would support spending something like 50% more for a ticket to only shave off 33% of flight time.