What people who haven't played quarterback...or really studied it...fail to appreciate it is this:
Quarterbacks have about 2 to 4 ticks to anticipate about 10 different variables...and they are throwing the ball...into...the...future.
A QB throws the ball in the now. But it arrives half a second or a second later...and in the NFL...the field can change a lot in that duration.
It's a billion times more complex than simply taking a sack or throwing the ball away...because you are dealing with tiny windows where a ball must go...and fractions of time in which that ball must be released.
NFL qbs rarely have the luxury of seeing a receiver wide open and throwing it to him in a spot where he will be wide open. QBs have to anticipate where the receiver will be--and where one, two, sometimes 3 defenders will converge.
When a QB is cut short on his time allotment...tons of bad things can happen. One scenario is that the receiver hasn't even completed his route and he's not turning back to see the ball...giving a safety an advantage to better position himself for a pick or a deflection.