Because in the later stage of fitness, the more you need the coordination of all your muscles to support it. For example, if you sit on a Russian swivel, although you exercise your abdominal muscles, because your legs are suspended, you need the support of your leg muscles, and the axial rotation of your back also needs the support of your back muscles, so there is no movement or fitness action that can be completed entirely by local muscles. If you don't train your legs and other seemingly unimportant muscles, the more difficult it is to exercise later, you will feel that your upper limb strength is passable, but you can't finish what you want to do.
Moreover, general fitness is a combination of aerobic exercise and anaerobic exercise, one is to reduce fat and the other is to gain muscle. In fact, many exercises in the process of reducing fat can actually exercise leg strength, such as running and swimming or women's aerobics, which can virtually exercise leg strength. If the leg strength does not increase after a period of fitness, it means that there is something wrong with your own fitness method, and blindly increasing muscles has no effect in terms of strength or aesthetics. So if you want to keep fit, make a perfect fitness plan and try to take care of all your muscles, especially the core group, so that you can go further on the road of fitness.