Their rocket exhaust as they’re being fired would slow the planes down, rather than providing a boost
For the same reason, cars have exhaust on the rear so that they can go much faster forward than backward
Edit: /s since you apparently need it after every joke. I know that missiles aren’t really fired, they detach and then propel themselves. Also, anyone who’s ever idled their car on a flat surface knows that the exhaust thrust cannot even overcome rolling resistance.