Because we don't select WRs in the draft. We select half as many as other teams.
Pick a team, and I'll do the comparison. Off the top of my head, the Bears (not exactly an offensive juggernaut): 23 WRs drafted during the Belichick era. The Patriots? 9.
We just don't expend resources drafting WRs.
Of those 9 we've chosen, only 4 showed ability to play (two were top WRs in the league, one was a good #2, and one had a career ending injury [Edelman, Branch, Givens, Mitchell]).
Then you look at Bears:
Good picks: Marty Booker, Alshon Jeffrey
Middling picks, backups: Justin Gage, Bobby Wade, Bernard Berrian
Young players, the book is not yet written on them: Anthony Miller, Darnell Mooney
Wasted Picks: Sulecio Sanford, D'Wayne Bates, Frank Murphy, Desmond White, David Terrell, John Capel, Jamin Elliott, Aerise Currie, Mark Bradley, Marcus Monk, Earl Bennett, Derek Kinder, John Knox, Joaquin Iglesias, Marquess Wilson, Kevin White, Dan Braverman, Javon Wims, Riley Ridley
23 picks: 2 receivers who are top WRs in the league, 3 backup or #2 types, 19 wasted picks. 2 guys who are young.
If Belichick had spent as many picks as the Bears have, I suspect he would have drafted at least 4 more solid WRs, if not more.