Belichick actually answered this exact question in a Press Conference one time. He said in effect:
"You can't gamble with First Round picks. If your wrong you set your program back too far, though some do. So I have to gamble with Second and even more with Third rounders to attempt to equalize the cumulative effect of always drafting toward the end of the round, that the League imposes to equalize talent. As with all gambles, sometimes you win, sometimes you must be prepared to lose."
That was one of those occasional very illuminating philosophical discussion questions that along with history of the League, that Belichick allows himself to answer. He does so only because they can provide not even the remotest possibility of an advantage to his opposing Coaches.
Commenters even speculated that was one of the reasons the Patriots maintain their own college draft scouting system, and do not rely on the National scouting services that many teams use. He is looking for the overlooked talents, or unique situations, that he can exploit in other rounds.
History tends to bear out his actions. In the BB era, the Patriots have never outright failed with a FIrst round selection, although some have not produced as much as hoped.
Meanwhile I do not accept your revisionist history that the Belichick Patriots are poor drafters. It is more than luck, it is talented players selected that have made them 10+ game winners for the last decade and a half.
