This is because the writers aren’t locked into the jokes that Ryan Reynolds actually uses on set.
Writing comedy can actually be a lot less fun than the finished product implies because you have to work hard to make sure every joke counts.
Because he’s wearing a mask, all his dialogue has to be re-recorded anyway, meaning the writers can always add in new lines if they come up with better jokes during post-production.
However, the Deadpool movies are actually easier to write than most.