I had a non-horse person ask me today why we didn't sedate a mare we want to move but who refuses to load.
There's a good reason why. Horses travel standing up. If you've ever had to stand on public transport, you know that you have to grab on to something and use some balance. A horse standing in a trailer may have four anchor points, but they can't grab on to something. So, they need to be alert enough to balance themselves, especially when the trailer or truck turns.
Occasionally you might indeed sedate a horse for transport - but there has to be a good reason. Very rarely horses are sedated for air travel (the stalls are narrower).
Generally, you would only sedate a horse if it was a sick or injured animal that you had to get to an animal hospital quickly.