The draft is inherently superior not just because of the talent, but also because of the value it can generate. If there was no salary cap and we could spend hundreds of millions of dollars more than everyone else, I could build the best team in NFL history.
But there is a cap, which makes value matter. As an example, Calvin Johnson is a better receiver than Demaryius Thomas. But I'd rather have Thomas for 5 years, $12.2M than Johnson at 5 years, $150.5M. Johnson is better, but not 12 times better. Adrian Peterson is the best running back in the game but 7 years, $96M is a lot of money to pay a running back with injuries. LeSean McCoy led the league in rushing last season and is under contract for 5 years, $45M. Better value. Alfred Morris finished 4th in rushing last season and is under contract for 4 years, $2.2M. Much better value. AP is the superior player, but not twice as good as McCoy, or 45 times better than Morris.
Obviously rookies are not predictable, proven commodities. But you get 7 picks every year for free. The cost will vary depending on the pick locations, and you won't hit on all of them, but the potential value is incredible. As an example, the Patriots went into the draft with 9 picks and a projected 1st year pool cost of under $5.3M. If only 2 of those picks work out, that would still average out to less than Kyle Orton made last season to play 1 game.
I don't disagree that trades or free agent signings can provide similar value. It's just much rarer and more difficult to find, and often much riskier. Moss is actually a great example of both the good and bad with trades. When the Vikings acquired him, they gave up Napolean Harris, their 1st round pick (7th overall), and a 7th round pick. Of course the Vikings blew the pick on Troy Williamson (who?), but it was still a lot for the Raiders to give up for not a whole lot. They were also paying Moss a lot of money as part of his 8-year, $75M contract, a particularly big hit when the team salary cap was $85.5M in 2005.
After 2006, lots of people including his coach were saying he was done, so we were able to acquire him for a 4th-round pick AND he reduced his salary from $9.25M to $3M. At that rate, the value was incredible and comparable to a rookie contract.
Acquiring a big-ticket free agent or making a blockbuster trade are like buying stocks at their peak, or paying sticker price for a new car. You're buying them at their best and hoping they continue to stay at that level. There's not a whole lot of value there, and you usually only do it because you need to.