If I haven't rode both bicycle and skateboards for distance for a while, I'd have exact same thought as you. From my experience of casual rider, it's opposite - I can easily go for hours on skateboard, while on bicycle I get tired and uncomfortable. I've ridden same bike paths 10-20 miles on bicycle and skateboard, and for me skateboard is easier. My guess because on skateboard I can change body position and muscles involved, can switch legs, push or pump. While on bicycle I'm locked in the same position, using same muscles all the time.

Jogging is way way harder than skate, I can barely jog 5 miles, but on skateboard 5 miles i won't even sweat.

There're various types of longboards and indeed some are quite high and you end up doing one legged squat. There many low to the ground models designed for distance specifically, that are much more comfortable to push, no squatting. Check https://www.instagram.com/p/Cbah6FrJuUv/ or https://www.gbomblongboards.com/longboard-skateboard-gallery

Cycling is more efficient that skateboarding, but surprisingly by a small factor. 24h record for cycling is ~900km, while on skateboard ~500km.