Because abs have been called out:
The body has a couple joints. They alternate between mobility and stability.
Ankles and wrists, shoulders, hips and the upper back are mobile joints. The muscles that move them are trained by moving weight.
Elbows, knees, lower back and neck are stable joints, the muscles that "move" them are trained by putting on isometric load.
You need to think about abs and its antagonist, the lower back muscles, this way: If you hold 50kg above your head, and you can still stand up right, whats the load on the abs? Lets say the distance from the hips to the rib cage is about 1 third the distance compared to that from the hips to the extended arms. That means the force your abs work against is about 150kg. You might not be able to get the weight up there because you cant do the press, but if it was placed there, youd be able to hold it up for a while.
If you do a situp, or any kind of stupid ab machine, youre maybe moving your head up and down 5-10 inch. Thats about 1 tenth the resistance of the 50kg and the 50kg wasnt your "max" on the abs to begin with. Situps train your abs at ~1% of your 1 rep max. Good luck achieving anything doing that.
A decent abs exercise is the squat. Another decent abs exercise is the deadlift. If you squat 1 times your bodyweight, you will have abs.