I am not saying you could not do that but what if those guys are not their. Or better, much better, players are there at other positions. Maybe no safety is worth a poo after the 2nd round. Or Jaelen Strong is taken by KC at 18, which is very possible. Then what do you do? As Gettlemen said "when you reach you get fuged".
You don't have to fill all you needs in FA, but if you don't at least have guys that can play ok at those positions and all the good players are gone at the position you need by the time you draft, what do you do then.