When it comes to young players starting their careers, we often don't know just what they'll turn into. A highly touted first round pick could meet expectations, or he could bust; a fourth rounder one may not give second thought to could fulfil his prophecy, or he could turn into a team's most valuable player.
There's no way to know offhand, and it takes time to find out just what you have. Repeat those steps over and over for every single prospect a team chooses or signs, and it's a never-ending cycle of questions to fill up a limited number of spots available.