Exhaust pipes must be snaked around different components underneath the vehicle in order to exit at the back, therefore, some bends are necessary.
Why there is a 90 degree bend in some exhaust pipes. What is the use of it?
I read your profile, so perhaps this link may be of interest to you.
https://www.reddit.com/r/AskEngineers/comments/2c6w7f/question_about_exhaust/
Air flows at different velocities within a pipe
cross section . For a well developed ideal flow , the velocity of pipe will be
higher near the pipe's center and decreasing towards the pipe wall in symmetrical
manner.