I think the difference is that existing players have already proven they can play at the level required, despite their character flaws, and some of those players you mentioned were top shelf, in terms of football ability.
So i can see why a club may take a risk, with the belief that they can turn around or at least keep in check whatever baggage they came with.
Sometimes it is the kick in the arse of being discarded by their club that they need, or at least thats probably what their new club is hoping.
When you talk about draftees, you still have question marks over whether they can actually make the step up and combine that with a questionable character, which decreases their chance of making the transition, and clubs are starting to see it as too risky.