Some have tried piloting and didn't get through; a few have a problem with that but most move on.
Some of them just don't get that kick from poling that pilots do and are quite content to work in the back seat.
And a few start off as backseaters and make the move into the front. In my experience some of these have made exceedingly competent pilots.
No (RAF) WSO actually gets any programmed flying lessons but I have to say that given a chance to pole from the back (on the rare day a 2-sticker is available) some are surprisingly good!