the simplest way to think about it is that your body is evolved to survive, which means it tries to use as few resources as it can get away with and stockpile the rest as fat to be burned if you run out of food.

Exercise is just forcing your body to realize it has to spend resources on muscles and stuff, but it'll still do the minimum it can get away with, so you gotta do exactly the things you want the body to build for.

If you just do 50 reps with 10kg you'll be able to do 50 reps with 10kg, but you probably won't be able to do half as many reps with 2x the weight, and you won't necessarily be able to do 2x the reps with half the weight.