|Where does energy go when you try to push a heavy object and it doesn't move? Thermal energy isn't made, so why do people get tired?
While it's true that there is no thermal energy made by static friction, since the object doesn't slide, your body can still make thermal energy directly. You get tired because your muscles must turn useful food energy into thermal energy whenever they are under tension. If you are doing work, they also convert food energy into that work, but even when they aren't doing work, they still convert food energy into thermal energy.