Why does the A-4 Skyhawk sit nose-up when on ground? The rear of the plane is nearer to the ground, while the front part of the plane is away from the ground (see image below).

Other planes are more flat.

I think it was for high AoA but these planes are carrier operated so I don't think there is much sense for that.


(wikimedia.org)
This was pretty common on naval aircraft from that era which used the launch bridle catapult interface. It gave the aircraft a high angle of attack during launch and helped the aircraft get clear of the bridle at the end of the launch stroke. It's noticeable on a wide variety of aircraft, including the F7U-3, F4D, A-5, F9F, F8, Super Etendard, Buccaneer, F3H, just to name a few.

Even the F-4 did. The nose landing gear oleo strut could be extended to give a nose high squat for cat shots:


(theaviationgeekclub.com)