Because aircraft carriers carry MANY aircraft (see above) and during flight operations (take-offs), they are staged on the flight deck behind the catapults. There’s no place else for them to stage.

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If you wonder about the Hanger Bay (a large area under the flight deck where aircraft are stored or maintenance is performed), it would take too long to bring each aircraft one at a time up to the flight deck and aft to position it on the longer catapult.

Normal flight operations is a time pressure business; launching aircraft as fast as they can. That’s why modern carriers have FOUR catapults so they can maximize the rate of aircraft launches.
File:US Navy 100703-N-6720T-029 Sailors aboard the aircraft carrier USS George Washington (CVN 73) spell out GW 18th on the flight deck while arriving in port to commemorate the ship's 18 years of Naval service since her commissioni.jpg

BTW, the two waist catapults (the angled deck) do start near the ships’ stern.

If all launches were from the stern of the ship using the full long length of the flight deck that would mean that the ship would then only have two catapults.

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