[Serious] Why are draft picks worth so much in trades?

As a long-time hockey fan, I've wondered this one for a long time. Why are draft picks so highly valued when they are far from being a guarantee? I mean, you're drafting some 17-18 year old kids, some straight out of high school, it's a huge gamble.

We've seen Stepan & Raanta being traded for a 7th overall. These two are established NHL players and are far better than most (read again: MOST) first rounders will ever be. It seems like GMs are betting on their pick to become the next big thing and are willing to exchange it for established players.

It seems like these days, with the gamble that are hockey picks, good teams are built through trading, which is why teams like Chicago and NYR have been able to remain consistently good for so long without really drafting a significant amount of prodigies. Teams seem eager to trade established players for even low picks, so why do GMs still have this massive hard-on for picks?

Sometimes you hear established players getting traded for 4-7th round picks. Statistically, a 7th round pick has extremely low chance of having any sort of significant NHL career (it does happen but you guys know the drill). It's mind-boggling, really.

Thanks!