If one of your picks hits, why would you deal that player away? And if the pick doesn't hit, how are you going to get the same value for that player if you start shopping him while he's still on his rookie deal? This is also after the rest of the league has gotten a good look at the player after his first 1-3 seasons in the league, meaning his value has been more accurately defined by each team's scouting department.
Also, if you draft a guy and then trade him too soon, it sends a bad message to prospects. It also looks bad because it suggests your organization made a mistake in that someone you trumpeted as being a good pick is now sent packing so soon. Fans don't want to be in perpetual rebuilding mode, and trading a recently drafted player for a future pick who will require another year or two to equal the same amount of experience is damaging to fan morale.
In the case of Sullinger or Olynyk and this year's draft, you're right in that it won't happen. One, the Celtics would not want to cut bait on players they've spent 1-2 years developing only to deal them away and start the process again with a rookie. Second, every draft is different. This draft is considered deeper than those previous drafts so saying "Sullinger would be 4th in a redraft!" does not equate him to a 4th overall pick this year. Third, the more years each player plays and is not a breakthrough hit, the more his ceiling is defined and GMs would rather take a new player who may have greater "upside."
Finally the years do matter. A good player on a rookie deal gives your team lots of value for the dollar, for 4 years. Sullinger's deal will be up in two years, compared to 4 for this year's class. That's 2 extra years you have cheap labor as opposed to losing a player entirely or having to potentially overpay them.
Why are you asking anyway? You feel Sullinger or Olynyk should be traded for picks in this year's draft? If that's the case, you must think they are overrated somehow, or at least not as talented as players in this draft class. If you do feel that way, consider the possibility that a lot of GMs feel exactly the same.