Throwing throwing the ball out of bounds without a receiver in the area is illegal, and is called when it happens (though NFL quarterbacks are usually smart enough to throw it close enough to a receiver that it's not going to be called very often).
Outside the tackle box, throws can be to nowhere. But inside the tackle box AND under heavy pressure, the call was easy.
Lots and lots of judgments involved - but Brady made this easy. It was grounding ... and it was a bad one.
Sure, without a receiver in the area, but when they make that throw “with a receiver in the area”, it’s 10 feet over everybody’s heads. Ain’t nobody catching that, not even Wilt Chamberlain. The intent is obvious.