There are so many other reasons why that might have happened. Sorry but regardless of your personal experiences with budgies, it doesn't change the fact that they're incredible flyers. Why do you think that a species of birds that depends on flying for survival is bad at flying as a species? That doesn't make any sense. There are plenty of reasons why budgies in captivity may be bad at flying, but in the wild the reasons are much fewer in number.

There are other reasons why your budgie might not be great at flying (wings clipped when young, not enough time outside of the cage, etc) but in the wild they are fantastic at flying and can turn on a dime.