The odds of getting a superstar through trade or free agency is, in my estimation, just as unlikely as doing so through the draft, especially if you aren't a glamorous location or in a tax-free state. At least through the draft you have the player for a longer period of time and see him develop throughout the years.
If the Sixers wanted a competitive team they could easily have one tomorrow. They have the assets to flip the switch because it's really, really easy to go from a bad team to a mediocre one. The hard part is going from a mediocre/above average team to a very good/contender level team, because that requires superstars and there's just not enough of them for all 30 teams.
Okafor, Saric, Noel, Covington, their draft picks, and even Embiid could all fetch veteran talent easily. If they wanted to field a starting lineup of Ty Lawson, OJ Mayo, Gallinari, Taj Gibson/Markief Morris, and Roy Hibbert they could do so tomorrow.
Young players with potential and superstars are the most valuable commodities in the NBA. Why? Because they're both underpaid. Everybody else is grossly overpaid by comparison and due to the salary cap the more underpaid guys you have, the more talent you can actually squeeze onto your team.
Put it this way: The Cavs are paying LeBron James 23 million dollars this year. The Sixers could easily sign two OK players 11.5 million a year, say an OK starting center and shooting guard. However, the production they are getting from those two players for the same money is nowhere near the impact LeBron, or any other superstar for that matter is providing. The problem is those stars are not available to the Sixers. So instead of paying average/above average players that money they're trying to get that superstar through the draft. Is that so hard to fathom?
Like Zach Lowe says frequently, tanking is no guarantee, it's just the best option out of a lot of unlikely options.