The whole process is kind of built against showing any solid plans until as late as possible. It's hard to argue against the nebulous idea of 'create more jobs' other than 'that's not an actual plan'. Once your opponent provides a solid plan you get all sorts of new ways to attack it like cost (especially cost). Another reason to not put out a detailed plan is that the actual bills/regulations you can get in office are going to look pretty different than what the base of $PARTY would like to see because you have to actually get it passed, but then you're not delivering on the promises and no one likes to hear 'we knew the proposal was ambitious and impractical to get passed but we told you we would to get your vote'.

The second reason is the correct one.

Downvoters, tell me why I'm wrong?