Because the front sight is placed over a hole in the barrel, routing gasses back to the bold carrier group to cycle the magazine. "Timing" an AR is figuring out how long the bullet takes from exiting the cartridge, to passing the gas hole, to exiting the barrel. Until the bullet leaves the barrel the gas passes through the hole, the sight base, gas tube and into the upper receiver to push the bolt back and cycle the gun is the only way for gas to escape (read: pressure, cause the bullet is still effectively plugging the barrel). The time from the bullet passing the gas hole to exiting the barrel (venting the gasses that were going back to cycle the bolt) is called the dwell time. A big design deal to get that timing just right.
Too close the the business end and there isn't enough gas routed to cycle the bolt. Too close to the back and there is way too much gas going in, over pressuring the system and gunking up the receiver. That's why mid-length AR carbines can be a challenge to get right.
An alternative on some rifles is to use a low-profile gas block and put a separate front sight further down the barrel. Longer sight radius without affecting the timing.