It doesn't matter why there was no receiver in sight. There wasn't one - and Brady was inside the tackle box - and he was under pressure.
Who's to say it would have altered outcome anyway? Psychic refs? he he he
Intentional Grounding of Forward Pass
Intentional grounding of a forward pass is a foul: loss of down and 10 yards from previous spot if passer is in the field of play or loss of down at the spot of the foul if it occurs more than 10 yards behind the line or safety if passer is in his own end zone when ball is released.
Intentional grounding will be called when a passer, facing an imminent loss of yardage due to pressure from the defense, throws a forward pass without a realistic chance of completion.
Intentional grounding will not be called when a passer, while out of the pocket and facing an imminent loss of yardage, throws a pass that lands at or beyond the line of scrimmage, even if no offensive player(s) have a realistic chance to catch the ball (including if the ball lands out of bounds over the sideline or end line).