Would you take on a job if you only got paid when the site reached a certain position (at the whim of the search engines)?
The SEO guy would most likely use link farms or spam bookmarking for a really quick fix.
Then, a couple of weeks after they have been paid, the site gets slapped for violating Webmaster rules and drops like a stone.
The next SEO person would have to work longer and harder to get the site back to it's previous position.
I can see your argument but most decent SEO people would not get involved.